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This stores the Android SDK tarball and gradle caches between builds
to speed things up.  Since the unpacked SDK gets unpacked very time,
updating the version is just a matter of changing the variable.  Since
only the gradle caches are stored, i.e. the jars and gradle binaries,
updates there will only add more files to the cache.

UI: disabled repo name if apk is incompatible



See merge request !226

Code Style : Changed variable names to be consistent with other variable names

variable names are inconsistent in other files as well. At some places it is `mVariableName` and in some places it is `variableName`.  

suggestion : rename all public variables to `publicVaraible` and all private variables to `mPrivateVariable` ?  

we can use http://udacity.github.io/android-nanodegree-guidelines/index.html

See merge request !225

Also update CONTRIBUTING, since we're now going to use three digits in
the version codes for the second part in each version.

Translators:

Adrià García-Alzórriz      Catalan
Adrià García-Alzórriz      Spanish
Benedikt Volkmer           German
Irvan Kurniawan            Indonesian
Licaon Kter                Romanian
Marcelo Santana            Portuguese (Brazil)
Massimiliano Caniparoli    Italian
Mladen Pejaković           Serbian
Mohamad Hasan Al Banna     Indonesian
naofum                     Japanese
Pander                     Dutch
Phạm Nguyễn Hoàng          Vietnamese
riotism                    Chinese (Hong Kong)
Tobias Bannert             German
Verdulo                    Esperanto
Verdulo                    Polish

Misc gradle, lint and markdown updates



See merge request !223

Ensure `getContentResolver()` is not requested from `null` object.

Should fix #554, however I couldn't reproduce this. At the very least, I fail to see how it is possible with this new change for a `null` to get passed to `AppProvider.Helper.categories`, whereas it could've before.

In the process, I also made the database query run on the background thread, then update the UI on the UI thread.

See merge request !224

Previously, it was not explicit that the `onCreate` happened to be invoked
in the UI thread. Now it is, due to passing `new Handler(Looper.getMainLooper())`.
Also, the categories are now loaded in a background task, and then the UI is
updated on the UI thread.

Also, don't call `getActivity()` in the separate thread. Instead, use the
`Activity` which we have already checked and ensured is not null.

The old /ci site is gone. Link to the builds page, which at least shows
something related to CI.

Use gradlew consistently. Also, fix its calling after the cd.

An F-Droid backup should contain all the app's data.

compileSdkVersion in the build.gradle file was already bumped, but we
forgot about this one.

Enhancement : Repo URL added to apk's on AppDetailsActivity.

Fixes issue #527.
Adds repo URL to apk's present on AppDetails.

Samples  
![sample](/uploads/0d603cf7242f6f45348c78009f811623/sample.png)![sampl2](/uploads/31efab104bd52cecc9f2ea1e923116c6/sampl2.png)

See merge request !222

Fixes issue #527.
Repository URL's now shown as Provided By domain.name/repo

Add PMD



See merge request !221

Translators:

ageru              French
Danial Behzadi     Persian
enolp              Asturian
ezjerry liao       Chinese (Taiwan)
Fr Translation     French
Hsiu-Ming Chang    Chinese (Taiwan)
Jonatan            Swedish
relan              Russian
riotism            Chinese (Hong Kong)

Unlike findbugs, PMD is very easy to configure. Should be very useful as
long as we set a correct ruleSet.

They all come in mvdan/fdroid-ci. Same as in the other two repos.

Bump target SDK and support libs to 23

Android 6.0 brought some changes that affect F-Droid client:

1. [Apache HTTP client removal](https://developer.android.com/about/versions/marshmallow/android-6.0-changes.html#behavior-apache-http-client)
2. [PackageInfo.REQUESTED_PERMISSION_REQUIRED removal](https://developer.android.com/sdk/api_diff/23/changes/android.content.pm.PackageInfo.html)

Closes #518.

See merge request !214

This is a temporary solution. All code that uses Apache HTTP Client
should be rewritten.

Also update Support Library components to 23.2.0.

This flag existed in Android 4.2-5.1 and has always been set. Stop
checking it.

CI: Use our docker image, print out test coverage



See merge request !220

Translators:

Adrià García-Alzórriz    Catalan
Adrià García-Alzórriz    Spanish
Ajeje Brazorf            Sardinian
Alberto Moshpirit        Spanish
Alex Kalles              Greek
jetamkadlec              Czech
Ldm Public               French
Licaon Kter              Romanian
Mladen Pejaković         Serbian
naofum                   Japanese
Olexandr Nesterenko      Ukrainian
Verdulo                  Esperanto
Verdulo                  Polish

Lint fixes and checkstyle update



See merge request !219

Due to fixes and improvements, some new checkstyle changes are due.