Add get_auth_token permission to allow credstore to call keystore2.

This CL adds a new keystore2 permission "get_auth_token"and grants this
permission to credstore which needs to call keystore2 to obtain
authtokens.

Bug: 159475191
Test: CtsVerifier
Change-Id: I1c02ea73afa6fe0b12a2d74e51fb4a8a94fd4baf
diff --git a/private/access_vectors b/private/access_vectors
index a02a2a8..1ab4f73 100644
--- a/private/access_vectors
+++ b/private/access_vectors
@@ -718,6 +718,7 @@
 	change_user
 	clear_ns
 	clear_uid
+	get_auth_token
 	get_state
 	list
 	lock