BlendMode VTS tests

BlendMode::Coverage and BlendMode::Premultiplied test fix

HIDL only worked on SRGB for this test and
as HIDL this also passes just the Coverage and Premultiplied
with the colorMode SRGB
see: https://source.corp.google.com/android/hardware/interfaces/graphics/composer/2.2/vts/functional/VtsHalGraphicsComposerV2_2ReadbackTest.cpp;rcl=ddbbf784a80979fd0b2d926a1e3cdb68d0a49b61;l=938

BUG: 216172840

Test: atest VtsHalGraphicsComposer3_TargetTest
Change-Id: Iec16fc112d3ab6ee37e55ca9f3fa760a439e52eb