Have aapt2 handle read/write flags on resources

This includes a new chunk type for holding entries that are flagged but
does not include new chunk data for storing all flag names at the
resource table level.

Test: FlaggedResources_test.cpp
Bug: 420720496
Flag: EXEMPT Aconfig not supported on host tools

Change-Id: Iaa6130c467385ae00161841a84d5094914bf90b3