Keystore 2.0: Add convenience API for watchdog module.

Also make use of watchdog toggleable with a feature flag.

Test: N/A
Change-Id: Ibdd14de09bfb31067313373805475a8998fd1c00
diff --git a/keystore2/src/lib.rs b/keystore2/src/lib.rs
index 11af9df..c04c4b0 100644
--- a/keystore2/src/lib.rs
+++ b/keystore2/src/lib.rs
@@ -47,4 +47,6 @@
 mod db_utils;
 mod gc;
 mod super_key;
+
+#[cfg(feature = "watchdog")]
 mod watchdog;