Allow read/write flags on xml elements

This changes aapt2 so that it now allows xml elements with the
android:featureFlag attribute even when that attribute is read/write (or
not found altogether). This also makes it so that attribute is removed
when the flag is read only.

Bug: 377974898
Test: Automated
Flag: EXEMPT Aconfig not supported on host tools
Change-Id: I330eefe897ab6dd1301b073d7f69443e3428b5b6