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
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Update language translations.

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Bumping version to 2.1.0
Upgrade to latest Android gradle plugin

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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.
disable passphrase creation on registration

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Closes #1726
new emoji drawer

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Closes #1746
Null check for cipher text to prevent NPE on decryption

Fixes #1703
Closes #1728
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Move default SMS and system import to "reminders"

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Closes #1730
Update BUILDING.md

Correction of a typo and a few style incongruences.

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Closes #1750
Fix emoji backwards compatibility recents crash

The old emoji drawer stored emoji with a .png suffix. Replace it during
list deserialization.
Add a backspace key to the emoji drawer
Remove gray placeholder squares while loading emoji
Occupy all vertical space for emoji grid

Fixes the recent emoji list leaving an empty gap at the bottom of the
drawer depending on the number of items shown.