Fix password field focus in keyguard

This fixes a bug introduced in Change-Id: I34b7db402401a824f463d35d7546c05dc2979243 where
the top-most view was allowed to capture focus in order to ensure the device handled volume
key events.

This resolves the issue by restoring previous behavior and ensures we still handle media keys,
regardless of focus.

Fixes bug 7676996

Change-Id: Id2d1200be81640e4b4b7b5e3a0af099d6fc2d259