V4L2DecodeInterface: Switch to GRALLOC allocator

This CL switches to output allocator to C2PlatformAllocatorStore::GRALLOC
that enforces use of C2PooledBufferPool without need of allocating a
separate allocator ID. For this reason V4L2AllocatorId::V4L2_BUFFERPOOL
is no longer used and is removed in this change.

Bug: 268301611
Bug: 268305422
Bug: 270329448
Bug: 238710012
Bug: 238390060
Bug: 270331759
Bug: 238709912
Test: CtsMediaCodecTestCases
Test: CtsMediaDecoderTestCases
Test: CtsMediaV2TestCases
Test: GtsExoPlayerTestCases com.google.android.exoplayer.gts.CommonEncryptionDrmTest#cencSchemeTypeV*
Change-Id: Ife2fbb6ac4ff2dc25e006792f8dad2d8ec6f4ac4