Add empty keystore2 crate

Adds an empty crate with a build config for libkeystore2 and the
keystore2 test.

Also add a TEST_MAPPING file setting up keystore2_test as presubmit
test.

Bug: 160623310
Test: keystore2_test
Change-Id: I2cda9a74a43422453cf5d1d44867107c9f0292bf
diff --git a/keystore2/TEST_MAPPING b/keystore2/TEST_MAPPING
new file mode 100644
index 0000000..70e3175
--- /dev/null
+++ b/keystore2/TEST_MAPPING
@@ -0,0 +1,7 @@
+{
+  "presubmit": [
+    {
+      "name": "keystore2_test"
+    }
+  ]
+}