rollback to appcompat v20

v21 appears to just not be ready for production given the bugs.
// FREEBIE
Don't completely disable GCM.

// FREEBIE
Updated translations.

// FREEBIE
Bump version to 2.3.1

// FREEBIE
The future is the other direction.

// FREEBIE
Add callback method parameter to axolotl decrypt functions.

// FREEBIE
Merge pull request #2 from WhisperSystems/master

commit whisper commits