Add mainline tests to presubmit.

This config will test the module from HEAD against a bse images (e.g. R,
S).

Bug: 185553751
Test: atest
Change-Id: I0ba9fe08ac56f08d154fceb884fac6d3df5f3b2f
diff --git a/TEST_MAPPING b/TEST_MAPPING
index cb7c12c..42b145d 100644
--- a/TEST_MAPPING
+++ b/TEST_MAPPING
@@ -1,4 +1,9 @@
 {
+    "mainline-presubmit": [
+        {
+            "name": "MediaProviderTests[com.google.android.mediaprovider.apex]"
+        }
+    ],
     "presubmit": [
         {
             "name": "MediaProviderTests"