Split bettertogether test case into router and session test cases

Created two new directories (session and router) and moved the relevant files into each. Then updated package names to point to the right places.

Bug: 349106078
Test: Run presubmit and validate that all of the tests are still passing in both new test cases.
Flag: EXEMPT refactor
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:30f8c5115fce29a77c4ba338f7ea3a95ff22ed00)
Merged-In: I7a748f2287b0d1c9d40c2627452b1a758ae59c8a
Change-Id: I7a748f2287b0d1c9d40c2627452b1a758ae59c8a
diff --git a/media/TEST_MAPPING b/media/TEST_MAPPING
index e52e0b1..6a21496 100644
--- a/media/TEST_MAPPING
+++ b/media/TEST_MAPPING
@@ -1,7 +1,10 @@
 {
   "presubmit": [
     {
-      "name": "CtsMediaBetterTogetherTestCases"
+      "name": "CtsMediaRouterTestCases"
+    },
+    {
+      "name": "CtsMediaSessionTestCases"
     },
     {
       "name": "mediaroutertest"