Decouple the display manager from the overlay manager

The display manager relies on resources that may or may not have been
modified via runtime resource overlay. Because the overlay manager is
initialized in parallel with the display manager, the two need to
synchronize their work so the display manager knows when it can rely on
its resources.

An upcoming change will hold off on threading the system until after the
initialization of the overlay manager. This means all overlays should be
setup before other services start, which in turn means no additional
synchronization is required. As a first step towards this, remove the
synchronization between the display manager and the overlay manager.

Also remove unused variable LogicalDisplay#PROP_MASKING_INSET_TOP.

Bug: 80150169
Test: builds, boots
Change-Id: I844de9e09eb1464ae112e1b480d21cf662a026e0