bump targetSdkVersion to 28

Here's what needs to happen to bump targetSdkVersion to the latest version: 28

• remove all <receiver> from AndroidManifest.xml and make them load in FDroidApp.onCreate().
• add ACCESS_COARSE_LOCATION permission for WifiManager.getConnectionInfo()
• implement one or more notification channels to post notifications to users
• update SwapService's use of startForeground() based on Context.startForegroundService()
• review use of Locale.getDefault()
• check whether Unknown Sources is enabled using canRequestPackageInstalls() https://developer.android.com/studio/publish/#publishing-unknown
• check that a valid ContentProvider is defined for the authority in all Uris
• separately check whether READ_EXTERNAL_STORAGE and WRITE_EXTERNAL_STORAGE are allowed
• request FOREGROUND_SERVICE for SwapService
• use versionCodeMajor attribute
• use getLongVersionCode() method for PackageInfo
• replace calls to deprecated, embedded BouncyCastle for ZIP/JAR signing

sources:

• https://developer.android.com/about/versions/oreo/android-8.0-migration.html
• https://developer.android.com/about/versions/oreo/android-8.0-changes
• https://developer.android.com/reference/android/os/Build.VERSION_CODES#O
• https://developer.android.com/about/versions/pie/android-9.0-changes-28
• https://android-developers.googleblog.com/2018/03/cryptography-changes-in-android-p.html

Timeline of various app stores: https://android-developers.googleblog.com/2019/02/expanding-target-api-level-requirements.html