Fix crashes running android.appsecurity.cts.EphemeralTest

- test whether the caller is an instant app, not
the target app of the notification (if they are
different)
- Allow apps that share an app id to post as one
another

Test: runtest systemui-notification
Test: android.appsecurity.cts.EphemeralTest
Change-Id: I6bf7d42adb821ef22455d01f27e00276c0483220
Fixes: 116799402
Fixes: 116637376