Modify the back stack and result code propagation in the screen lock
scenarios.
- EncryptionInterstitial now propagates the result of ChooseLock*
request instead of always returning RESULT_OK.
- ChooseLockGeneric now treats CHOOSE_LOCK_REQUEST and
ENABLE_ENCRYPTION_REQUEST the same (since encryption can be a proxy
for ChooseLock*). This means ChooseLockGeneric will now stay on
back stack when going back from ChooseLock*, just like the case
(indirectly) through EncryptionInterstitial.
Bug: 26177240
Change-Id: Id7f1256dcbff00d552a3e7db60c285f53f1e63e6
e9d2abca86