Update lockscreen logic for media

Previously, media was using a combination of legacy state flags to
determine when lockscreen was visible. Keyguard transitions
replaces all this, and vastly simplifies the calculation.

Fixes: 408885411
Test: atest MediaHierarchyManagerTest
Flag: EXEMPT bugfix
Change-Id: I32a3d623dfd58fef3a386d9bccd0e5235e0729d9