Add APIs to control the external display state

External displays start disabled, signal connection / disconnection and
can be enabled / disabled by system apps.

The implementation of the signalling is mostly in LogicalDisplayMapper.

The flags are accessed through DisplayManagerFlags, which is injected
into DisplayManagerService. All other uses come from
DisplayManagerService and is propagated. This allows for injection in
tests.

Currently, we are using a standard flag, but if DeviceConfig is not
ready, we instead read a default static value. The value cannot change
during the runtime of the device.

When read only flags are ready, we can simplify the code to remove the
default value, as the flag will always be available.

To test on a local device, set
`DisplayManagerFlags.DEFAULT_IS_CONNECTED_DISPLAY_MANAGEMENT_ENABLED` to true.

Bug: 280739508
Test: atest LocalDisplayAdapterTest
Test: atest DisplayManagerServiceTest
Change-Id: I91411a8e726982d88474cf2ecd0f423bde9be107