Add DocumentsUIGoogleTests into postsubmit.

Using TEST_MAPPING file to configure postsubmit test.

Bug: 139258941
Test: Change to DocumentsUI folder, run command "atest :postsubmit"
Change-Id: I297b9d35e1c6967e38c187369f6a365afe88ddda
diff --git a/TEST_MAPPING b/TEST_MAPPING
new file mode 100644
index 0000000..98c2fc8
--- /dev/null
+++ b/TEST_MAPPING
@@ -0,0 +1,7 @@
+{
+  "postsubmit": [
+    {
+      "name": "DocumentsUIGoogleTests"
+    }
+  ]
+}