Allow keyguard_message_area_container to be null

In certain applications, the keyguard_message_area_container view may
not be present around a KeyguardMessageArea instance. Allowing the
container to be nullable and adding the appropriate null checks will
prevent exceptions under these circumstances.

Bug: 213476245
Test: manual
Change-Id: I65a4b6577c7b420ce6a55ae1d459989855c2e7a0