[rkpd] Refactor rkpd_client into an independent lib for reuse

This allows rkpd_client to be reused by both keystore2 and
AVF pVM remote attestation.

Test: atest keystore2_test librkpd_client.test
Bug: 241428146
Change-Id: Ibdf95c4deb2ba499daaecd170c2971cda4e80bba
diff --git a/keystore2/TEST_MAPPING b/keystore2/TEST_MAPPING
index 1038bea..57ce78c 100644
--- a/keystore2/TEST_MAPPING
+++ b/keystore2/TEST_MAPPING
@@ -35,6 +35,9 @@
       "name": "keystore2_client_tests"
     },
     {
+      "name": "librkpd_client.test"
+    },
+    {
       "name": "libwatchdog_rs.test"
     }
   ]