Fix cross-user background activity starts.

This fixes some issues with checking background activity starts when
there are multiple users:

1. System server can run in other users, and we need to take this into
account when checking for Process.SYTEM_UID.

2. Starting an activity via cross-profile intent uses
startActivityAsCaller. This will generally fail our checks (because the
app we are pretending to be is not foreground) but it should be
allowed - when we use this call the system server definitely wants the
activity to be shown, and there are sufficient permission checks in
place to prevent abuse.

Bug: 130622120
Test: CTS Verifier tests no longer get blocked activity starts.
Test: atest BackgroundActivityLaunchTest
Test: atest RootWindowContainerTests
Test: atest WmTests:ActivityStarterTests
Test: atest CtsWindowManagerDeviceTestCases:ActivityStarterTests
Test: atest CtsAppTestCases:.ServiceTest
Change-Id: Id05a2cb089ac99b15cff8a3cd311ee043f13c8e1