Add composer API changes in accordance to partner feedback

* Add a DimmingStage to the client target configuration reported by HWC,
  which may be used for some vendor color modes to configure when
  dimming should occur during client composition
* Communicate the display brightness in nits, so that HWC
  implementations do not need to parse the per-display xml to map from
  display brightness to nits. Furthermore, this is more plausible for
  extensibility for external displays.

Bug: 220396224
Bug: 218954037
Test: builds
Change-Id: I5675c16f0895f9958e3bee3ee4c85df8937ecdb7