[flags] Skip nested disabled manifest elements

Original manifest parsing code used to just move over from
the flag-disabled element to the next one, missing that the
next one could be nested. Now it properly skips to the following
element on the same nesting level

Bug: 417820920
Test: atest PackageParserTest
Flag: EXEMPT bugfix in flagging
Change-Id: I5b5e85541e5e54c3a47bd70ebb45febd93569e9e