Partial revert of ag/7508382 to retain back button responsiveness in BiomtericPrompt

The layout should be focusable and be set to focused in order for the
back button events to be received, since the key listener is set on
the layout.

Fixes: 133781042

Test: Back button cancels authentication
Test: No regression in tests mentioned in the CL above
Change-Id: I7d78bea400e8791e296a53603414095cfca62ec1