Add support for .flat files with flags in the directory

aapt2 allows flagging the resource file by specifying the name of the
flag in the directory, such as
`res/flag(test.package.myFlag)/values/bools.xml`, in which aapt2 will
generate files like `values_bools.(test.package.myFlag).arsc.flat`. This
change adds support for such aapt2 generated flagged resource files.

Test: Patch ag/29931886 && ag/29501772 && m
Bug: 374827548
Change-Id: I0ea6d6f747ad008c8f2a554a192196dc59f0c1a8