DisplayManagerGlobal: suppress redundant onDisplayChanged events

With Frame Rate Override enabled, an app could get an onDisplayChanged
events which seems redundant. An example would be:
1. Display changed refresh rate to 60Hz - onDisplayChanged was called
2. App's frame rate override changed to 60Hz - onDisplayChanged was called

To avoid sending these redundant events, DisplayManagerGlobal caches
the last DisplayInfo that was reported to the app and based on it it
decides whether to send the event or not.

Bug: 184588343
Test: atest SetFrameRateTest
Test: atest FrameRateOverrideHostTest
Change-Id: I97c7b2b9a799424998b1b717a4311d3d08b3178f