Update get_sched_policy to return TOP_APP correctly.

Currently get_sched_policy() would only return foreground
or background, but not TOP_APP, since it's not looking
at CPUsets at all.

Fixing this makes testing easier, and it also corrects
the output of "ps -P" to show "ta" for current top app.

Bug: 27857142
Change-Id: I589cacc107f355de15750f01b06f404b59a0f99b