Migrate from ActionBarSherlock to AppCompat

This is necessary for moving forward with Android's common UI components. Also, it simplifies our multiple PasswordRequiredSherlockActivities as all ActionBarActivity objects support the compat fragment manager.

This also uses the support-v4-preferencefragment library to also include support PreferenceFragment objects, since AppCompat fails to include it.

Most of the changes are namespace replacements.
From: https://github.com/WhisperSystems/TextSecure/pull/1662/files#diff-d1328da0a7bdc00dd18b5bc726a8bdabL109