hwc2: add client target support

Client can set the client target buffer that will contain all the layers
composed by the client

Test: Add "TARGET_USES_HWC2 := true" to BoardConfig.mk.
      Recompile.
      Run testcases: https://android-review.googlesource.com/#/q/project:
            platform/frameworks/native+branch:master+topic:test-hwc2

Change-Id: I638db7408b1e878643acef0f3fa8895ba428b1ea
4 files changed