Extend VTS to support multiple display if available.

[Why]
In efforts to extend support for connected displays (or even devices
with multiple displays such as foldable devices), extend the same VTS
tests we have but make them run on all discoverable displays.

[How]
Nothing fancy is going on here. Instead of running the tests on
mDisplay[0] and use the primary display ID and dimensions, simply loop
over all available displays and run the tests against each of them
independently of each other.
To make the review easy, only *_TargetTest.cpp is changed.

Bug: b/391347760, b/355482219
Test: atest VtsHalGraphicsComposer3_TargetTest
Flag: TEST_ONLY
Change-Id: I65d7fae47d486121b047a7c888db11c4842b4640