Otapreopt: Filter preopted system apps

The tool will receive requests to compile preopted system apps, as
we can't detect changes right now and need to be conservative. As
some apps cannot be stripped (signed before being preopted), they
can be re-compiled on device, producing stale files that won't be
cleaned up.

Add code to detect this situation and avoid a compile.

Bug: 25612095
Change-Id: I5f489e61b8d4e5b619cbaa01f6c59880479a46cd