Add xxhdpi check drawable for notifications

Fixes blurry mark as read icon on Android Wear. FREEBIE.
Merge pull request #1678 from veeti/check-icon

Add xxhdpi check drawable for notifications
use latest android number as recipient number

Fixes #791
// FREEBIE
tests
more tests, fixed race conditions
address comments
make tests a bit more succinct, add some fuzzy number comparison tests
move isNumberAddress
async db updates, concurrenthashmap
address comments
remove threading, simplify cache logic
remove unnecessary db call, move cache logic