Add test mapping for ScreenshotHelperTests

Runs tests on presubmit


Test: Ran `atest` in core/tests/screenshothelpertests,
`atest` successfully picked up screenshothelpertest as
a presubmit test, ran it, and the test succeeded. Test
output available here:
https://drive.google.com/open?id=12M28e84bkAZtnQYe7-6f8ybs31bULWV_
Bug: b/140526005
Change-Id: Ic0a288c88232eeddd03591b590b0ef3d8c98fa93

core/java/com/android/internal/util/TEST_MAPPING

@@ -0,0 +1,7 @@
+{
+  "presubmit": [
+    {
+       "name": "ScreenshotHelperTests"
+    }
+  ]
+}