Vts readback status check fixes

When we query for the getReadbackBufferAttributes
we can get EX_UNSUPPORTED error when not supported
and when this happens readback should not be supported.
EXPECT_TRUE for this failure case
was causing the test to fail when we received an EX_UNSUPPORTED
 for this.
See for documentation: https://source.corp.google.com/android/hardware/interfaces/graphics/composer/aidl/android/hardware/graphics/composer3/IComposerClient.aidl;rcl=e1f0950945f687ed374a45911e2d7786de724ca4;l=436

BUG: 221889462
Test: atest VtsHalGraphicsComposer3_TargetTest
Test link for the cf_x86_64_phone-userdebug  http://ab/I39800010022763398
All the other test failures are tracked else where.

Change-Id: Idf352761c5cc808540edfe2042c5ee4602b5e96b