hwc2: set layer blend mode

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: I490c5e06403c295aca7e1b6855269bf90f2891be
7 files changed