Add FrameRateOverrideMappings unit test

- Add unit test
- Modify Scheduler and FrameRateOverrideMappings to check
  supportFrameRateOverrideByContent in getAllFrameRateOverrides
- Remove flag usage in Scheduler to keep it within
  FrameRateOverrideMappings class
- Add operator== for FrameRateOverride event to enable ASSERT_EQ

Bug: b/204322816, b/216445833
Test: atest libsurfaceflinger_unittest:FrameRateOverrideMappingsTest
Change-Id: Ie8717e76cb1bcfeefac93a7093ee30581767c18b