[VTS] Update allocator name for AIDL allocator.

We used "default" value for the allocator in the test
and this value was used as is for the AIDL allocator.
This caused the test to fail because we were not able to acquire the
appropriate allocator as AIDL allocator instance is not named "default".
Update the test to use the correct instance name for AIDL allocator,
if available.

Test: atest VtsHalGraphicsComposerV2_1TargetTest
atest VtsHalGraphicsComposerV2_2TargetTest
atest VtsHalGraphicsComposerV2_3TargetTest
atest VtsHalGraphicsComposerV2_4TargetTest
atest VtsHalGraphicsComposer3_TargetTest
BUG: 234671596

test results after updates:
VTS 2.1 : http://ab/I84600010058657636
VTS 2.2 : http://ab/I51800010058498973
VTS 2.3 : http://ab/I87700010058914863
VTS 2.4 : http://ab/I88900010058592031
VTS 3.0 : http://ab/I96200010058838905

Change-Id: I04ae1a18d757cfd941d4929ad08f6bb2c8643f76