Add special lock screen prompt strings for strong auth.

Due to incorrect strings, we temporarily disabled the prompt strings for
strong auth and instead used the generic ones as a short-term fix. In
this CL, we correct the strong auth prompt strings and add them back to
the lock screen UI. The new strings are in line with those in keyguard.

Bug: 36511626
Test: manual
Test: make RunSettingsRoboTests
Change-Id: Ifba689db37cc7d331eb1a774814f6b6235977ff9