Merge: Prepare for the android UI module

* Not all pthreads features are supported on Android, thus the changes.
* Changing the `data_store` signature is enough to support nulls because the APIs of both underlying implementations already supported them.
* Declaring the lines for <activity> in the modules allows for non-fullscreen and non-orientation-locked apps!

Pull-Request: #927
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Jean Privat <jean@pryen.org>