VTS: Update ReadbackTest to run on all available displays.

With no functionality change, change all tests to run on all available
displays one at a time.

Bug: b/391347760 , b/355482219
Test: Create CVD with multiple displays then run `atest
VtsHalGraphicsComposer3_TargetTest`
Flag: TEST_ONLY

Change-Id: I6f5ab6f61498f871dedbfa3006dba0cdc9fcbd49