Bump to Android SDK version 24 (Android 7 - Nougat)

[tishun]

We are currently stuck on version 19 (20 if you ignore the Fragment issues).

This also makes it hard to move to the latest Facebook API.

The major obstacles are that the StreamPhoto and StreamPhotoItem classes are deprecated and removed, but are still used in the NotifierService and ThumbActivity.

We need to:
a. Understand what are they used for
b. Remove them and replace them with similar logic

[tishun]

Removed them entirely. Leaving open until tests show what I've broken.