[refactor] Split watchdog in a standalone library for reuse

This simplifies the task of creating an independent library of
rkpd_client later.

Test: atest keystore2_test
Bug: 241428146
Change-Id: I2834c9be9f5100d52829e6392f0dd48e7c76beb1
diff --git a/keystore2/TEST_MAPPING b/keystore2/TEST_MAPPING
index f8a1302..1038bea 100644
--- a/keystore2/TEST_MAPPING
+++ b/keystore2/TEST_MAPPING
@@ -33,6 +33,9 @@
     },
     {
       "name": "keystore2_client_tests"
+    },
+    {
+      "name": "libwatchdog_rs.test"
     }
   ]
 }