When devices' clocks are out of sync, it is possible that a response is
shown before the request. This commit makes sure that the timestamp of
responses is always later than the last message in the conversation.
Some wording could be misunderstood to thing introductions were
successful even though they were not. That has been clarified.
A new database transaction was created when getting contacts and local
transport properties. This has been changed to re-use the existing
transaction.
Also addresses minor issues found in review.
The button hides itself when you scroll down the list of contacts
and shows again when you scroll up.
To properly color the button, the accent color has been defined.
It uses the same color as the action bar (primary color).
I leave it to a UX designer to adapt the color scheme.
Please note that the design support library was used.
It includes the app-compat library, so this has been removed
from the `build.gradle` file.
Closes#199
