Add apex TEST_MAPPING to frameworks/av/apex.

To make sure we catch certain breakages related to the media codec
(test) APEX at presubmit.

Bug: 129996486
Test: run atest in frameworks/av/apex
Change-Id: Icf5933aee02ebad80c709ec3de340bbc733a14bd

apex/TEST_MAPPING

@@ -0,0 +1,7 @@
+{
+  "imports": [
+    {
+      "path": "system/apex/tests"
+    }
+  ]
+}