Add TEST_MAPPING for build/soong/apex

build/soong/apex is highly coupled with system/apex.
So it is reasonable to "import" presubmit tests from system/apex.

Test: atest --test-mapping build/soong/apex
Bug: 138984456
Change-Id: I7447e04a63b86486daef0f2a7642842101c0029f
diff --git a/apex/TEST_MAPPING b/apex/TEST_MAPPING
new file mode 100644
index 0000000..d0223d1
--- /dev/null
+++ b/apex/TEST_MAPPING
@@ -0,0 +1,7 @@
+{
+  "imports": [
+    {
+      "path": "system/apex/apexd"
+    }
+  ]
+}
\ No newline at end of file