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.