Ensure keyguard stays hidden when transitioning between hidden states.

Defines hidden states which should hide the keyguard when we are fully
on that state, and also if transitioning between hidden states. This
ensures we never show the keyguard when transitioning between the dream
and glanceable hub.

Fixes: 359758497
Test: atest KeyguardRootViewModelTest
Flag: EXEMPT bugfix
Change-Id: If5c0e53e84dcc7eeed9a867110317c2fbae9204c