Rename sdkext module to sdkextensions

It's a better name.

Bug: 137191822
Test: presubmit
Change-Id: Icc39779d49bf31f911f9f72bd7610e84c5f49466
Merged-In: Icc39779d49bf31f911f9f72bd7610e84c5f49466
(cherry picked from commit 5b97269cffa587d1c1d73163a9cabb0cc8c7ec05)
diff --git a/TEST_MAPPING b/TEST_MAPPING
new file mode 100644
index 0000000..91947f3
--- /dev/null
+++ b/TEST_MAPPING
@@ -0,0 +1,7 @@
+{
+  "presubmit": [
+    {
+      "name": "CtsSdkExtTestCases"
+    }
+  ]
+}