Fix buildbots hidden_api_test

Target buildbots set /system to be the runtime module root because they
are running without an apex. The test assumes that /system dex files are assigned
to the application domain but here they get assigned to the core-platform domain.

Adjust the domain setting logic to check if the runtime module root is
distict from the android root (as a proxy for 'is running with apex').
If not (as is the case with the buildbots), skip checks against apex
locations.

Test: m test-art-target-gtest-hidden_api_test
Change-Id: Iff3890ec69cb04a1e4ed5bc2a3b5c652ada05f36