Don't call platformCompat on system apps.

To avoid boot time regression in tests, as compat doesn't care about
system apps.
Also fix the fallback condition to be correct.

Bug: 30188076
Bug: 142558883
Test: atest google/perf/boottime/boottime-test -v - no binder calls,
no time regression.

Change-Id: I15fd39ad7e0ed70fcd880a13cffa40d70c5bba55