IDataSource: add getFlags() to expose DataSource::flags().

This will let MPEG4Extractor cache stbl data. Therefore it can avoid data
flushing in data source (NuCachedSource2) due to reading stbl and access
unit data alternatively when the stream is larger than max cache size.

Bug: 26533748
Change-Id: Ia534755ab9130e4dcee94d53ca3c933d1b9eb566