Update getConnectedDisplays to generic getDisplays with extra field

Filtering out non-connected displays should be business logic that is
done on caller side. This unifies existing getConnectedDisplays() and
getAllDisplayIds() into a single getDisplays() and processed
accordingly.

Bug: 419742776
Test: atest DisplayTopologyPreferenceTest
Test: atest ExternalDisplayPreferenceFragmentTest
Test: atest ExternalDisplayUpdaterTest
Test: atest ResolutionPreferenceFragmentTest
Flag: EXEMPT refactoring util
Change-Id: If2e9f410469b09f6558b4d1a4d2c5f126ac4fbdc