Add AID_EXT_DATA_RW / AID_EXT_OBB_RW GIDs.

These will be used as a GID for the Android/data and Android/obb
directories respectively, on devices that don't have sdcardfs.

Bug: 146419093
Test: builds
Change-Id: I52a3235e872eb4b0402f918d5c47661f16626ba6