Add SelfTargeting test plan

This patch adds a trigger. The trigger is triggered when someone
changes the followin files
* core/java/android/content/om/*
* core/java/com/android/internal/content/om/*
* core/jni/*Overlay*

The trigger triggers the following tests to run.
* SelfTargetingOverlayDeviceTests
* CtsContentTestCases
    * android.content.om.cts
    * android.content.res.loader.cts

Bug: 205919743
Test: atest --collect-tests-only -p \
      frameworks/base/core/java/android/content/om/
Test: atest --collect-tests-only -p \
      frameworks/base/core/java/com/android/internal/content/om/
Test: atest --collect-tests-only -p \
      frameworks/base/core/jni
Change-Id: I6018d5806c93b7b0a87331a95e144a6ab45c068c