libmediatranscoding: Reorganize test assets

Move test_assets filegroup to correct Android.bp

Move test files in assets to a subfolder as needed in
AndroidTestTemplate.xml

Bug: 173627260
Test: Builds

Change-Id: I1e32081a069c4c1b25aa200dcdb165c0feb07b8a
diff --git a/media/libmediatranscoding/tests/Android.bp b/media/libmediatranscoding/tests/Android.bp
index 7b15b1b..e49df35 100644
--- a/media/libmediatranscoding/tests/Android.bp
+++ b/media/libmediatranscoding/tests/Android.bp
@@ -1,4 +1,10 @@
 // Build the unit tests for libmediatranscoding.
+filegroup {
+    name: "test_assets",
+    path: "assets",
+    srcs: ["assets/**/*"],
+}
+
 cc_defaults {
     name: "libmediatranscoding_test_defaults",