Create two Gradle flavours, "standard" and "foss".

This change creates two different Gradle flavours:
  - standard: Telegram as was before this change.
  - foss: Telegram without HockeyApp and Google Play Services.

To make future maintenance easier, I've implemented two service
wrappers (HockeyServiceWrapper and LocationServiceWrapper), intended
to remove build time dependencies without maintaining separate copies
for both ApplicationActiviy and ChatActivity.