Ensure TalkBack says "back" for back button

Change the description of the action bar back button from the default
"navigate up" to "back". Also fixes bug that caused title to be stuck as
"all permissions".

Test: Turn on talkback, and select the back button on the action bar on
any screen of the permission controller. Talkback should say "back
button".
Fixes: 147649455

Change-Id: Ifeff5522b12f16fe5415b3c2374ec0a5b306c2a6