RESTRICT AUTOMERGE : Returning HWC2_CAPABILITY_SKIP_VALIDATE capability if device supports it

For reasons unknown, HWC2_CAPABILITY_SKIP_VALIDATE was never added to
the IComposer hal.  However, we would like to tell the client that the
device has this capability when it is returned during a getCapabilities
query.  We manually add it to the list of IComposer::Capability
(static_cast should be ok since the IComposer::Capability has a fixed
underlying type of int32_t, and HWC2_CAPABILITY_SKIP_VALIDATE = 4 is in
this range).  Checking that device has skip validate functionality in
relevent test as well

Bug: 140813883
Test: build, boot, VtsHalGraphicsComposerV2_1TargetTest
Change-Id: I5f28268c4e6edeb2895502c5f4e009cebb566100