Fix apex_test.go and add it to Android.bp

apex_test.go wasn't listed in the Android.bp file, which allowed
it to bitrot.  Make the API level methods take a PathContext
so that they can be called from a test using configErrorWrapper.
Also fix an int that was converted to a string.

Test: apex_test.go
Change-Id: I1ff87134c837bd5d344d22550baabde10d1b0b2e