Add unit test for sdk library permission xml file

Followup to the fix in https://r.android.com/3137737. This adds a unit
test to check the permission xml file contents of sdk library. This file
contains the on-device location of the sdk implementation library.
override/overridden apexes are installed in the same location, so the
on-device sdklibrary path should remain consistent in the xml files of
the override and overridden apexes.

Test: go test ./apex
Bug: 346486531
Change-Id: I61adbf94bdfce8aa89de89cc90b6f564f2f7417f