OMS: add TEST_MAPPING

The tests for the overlay manager are comprised of

  - FrameworksServicesTests (part of)
  - OverlayDeviceTests
  - OverlayHostTests
  - CtsAppSecurityHostTestCases (part of)

Add a TEST_MAPPING file to track all of the above.

Bug: 130257644
Test: atest --test-mapping $ANDROID_BUILD_TOP/frameworks/base/core/java/android/content/om/TEST_MAPPING
Change-Id: I401179709e000bf6b41af99d548be732b9a03a34