[SurfaceFlinger] add getDisplayedContentSamplingAttributes i/f

Add interface to ISurfaceComposer that can query the
graphics.composer for querying the displayed content sampling
engine's supported attributes.

Bug: 116028618
Test: Boot
Test: ran test client against prototype, see attributes reported
Test: Ran new test './libgui_test  --gtest_filter="DisplayedContentSamp*"'
Test: on hwc with and without new function hook.
Change-Id: Ie4bebd98d134e63a9f3e21f79ae519a1fe8055b0