Use constant for no process

The value -1 is passed in as UID by non-PendingIntent call flows to indicate
that there is no actual process in the role of the real caller. This
just improves readability to replace those -1 values that are compared
to the passed in UID (and where we pass in a -1 UID to other methods).

Test: atest com.android.server.wm.ActivityStarterTests android.server.wm.BackgroundActivityLaunchTest
Bug: 306059525
Change-Id: Iafa0b45bb9b382115a581f5a90419177e352767a