Have test_current be a union of system and test APIs

Include @SystemApi APIs in the test_current stub jar, while keeping
the @TestApi and @SystemApi API txt files tracked separately.

A side-effect of this change is that string constants of @SystemApi
permissions are no longer inlined in the test api tracking, possibly
because metalava was previously incorrectly peeking at @SystemApi
whereas it is now explicitly forbidden to do so. I don't believe the
API txt change has any practical effects.

Bug: 129133308
Test: presubmit
Change-Id: I23d3c88624ee7599485c3390a0440b78bc18a0a2