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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.

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

Also update Support Library components to 23.2.0.

Hans correctly pointed out all this does is try to kill the echo. The
AVD should be taken down along with the docker runner.

I'd do 2.10, the current stable, but the android plugin seems to break
with it.

It doesn't include all of the Android style checks as found in Android Studio,
but it's a start.

Bump Gradle to 2.7 because the checkstyle plugin in earlier versions is just
not good enough.

It doesn't specifically apply only to the client.

Its boot animation is "stopped" but on my laptop the CPU is still at 100%,
which means it's probably still working and not ready yet. The tests fail if
you run them right after wait-for-emulator is done. If you wait a few seconds
though, they do work since the CPU load drops.

Until we can figure out a better solution, have a generous 30-second sleep
after it tells us it's ready.

It sometimes fails with the default (2 minutes), especially on the 4.4
emulator. Increase to 8 minutes.

This will make sure that all projects in the repository are built and tested.
Should come in handy for !126, which splits up the app into two apps and one
library.

Using 2.3.7 (10) armeabi for now. We can look into using more images in the
future.

Spotted by Hans at fdroidserver. The 'update sdk' tool will download the
package index at each run, so installing one package at a time will be rather
inefficient.