Add TEST_MAPPING file for postsubmit

Adds the TEST_MAPPING file so the CoreAppJankTestCases will run in
postsubmit. These tests confirm basic behavior for Detailed Application
Jank Tracking.

Bug: 382781252
Test: Presubmit.
Flag: EXEMPT config change.
Change-Id: I63fb31e2bfd31e23f9bdccc469cb0d08cf6ee9ad