Spelling and Android Style Guide changes

In addition to changing case according to the Android style guide (http://developer.android.com/design/patterns/index.html), wording and spelling errors have been fixed for the following strings:

RegistrationProgressActivity_this_number_is_already_registered_on_a_different
KeyExchangeInitiator_youve_already_sent_a_session_initiation_request_to_this_recipient_are_you_sure
MmsDatabase_mms_pending_download
registration_progress_activity__if_you_are_connected_via_wifi_its_possible_that_there_is_a_firewall
preferences__also_vibrate_when_notified