Gralloc: AIDL HAL may replace HIDL HAL.

Relax the requirement on compatibility matrices
that HIDL allocator HAL must exist on devices with
target level 3~6, because they may have the AIDL
Gralloc HAL instead. Use DeviceManifestTest.GrallocHal
to enforce the requirement.

Also fix comments.

Bug: 232003062
Test: TH
Change-Id: Ia00a9bdb15a39ebeb1ad74f3c922fe7923d7542d