Fix: Add messages to Notifications for Android Auto on Android M and older

In the previous implementation for Android Auto, messages are only added
on Android N and above. In order to support older Android versions, the
messages need to be added always.

Update strings.xml

Added a comma.

use pgp key from contact if not set in muc
fixes #283

Merge branch 'android-auto-support' of https://github.com/morgenroth/Conversations into morgenroth-android-auto-support

Add content description to send button

Add reply button in wear design

The existing reply button for Android wear has been re-used from the action
bar. Instead of a 'send' icon, a 'reply' icon better matches the intended
action. Moreover, action icons for Android wear are solid white instead of
half transparent.

This can be used by screen readers and other accessibility aids.

fixes #2444