Unify XML parsing and fix problem with AARs

Up until now, libraries never needed PSI parsers since they were read
only. aapt:attr attributes parsing is implemented in the PSI parser so
now, in order to support them, we need to ask for a PSI parser even for
read-only files.

Test: Existing tests pass
Change-Id: Idfbe5fd7ce79ac4d254e615a6d439dc6a2b5de16