Upgrade to latest Android gradle plugin

// FREEBIE
Closes #1660
Add date to saved media filenames

Fixes #1689
Closes #1693
Add number of messages to the notification number attribute

Although not used by stock Android, many custom ROM's (and possibly OEM
versions?) have a setting to display the "number" count of a notification
overlayed on the status bar icon. Add support for this.

Closes #1637
Create a Curve25519 asymmetric master secret for users without.

Fixes #1701
Bumping version to 2.1.1
Upgrade to new GCM API.