Use CTS variant of plat_file_contexts

The default plat_file_contexts may contain entries that are specific to
-userdebug builds. These entries are not found on release builds which
will trigger a failure of testAospFileContexts.

Note that the test expect the same entries or a subset. It means that
-userdebug builds with the extra entries described above will pass as
well.

Bug: 394725259
Test: atest CtsSecurityHostTestCases
Change-Id: Ia8de67a66f3c83616161b988437e073d533fb65c