Keystore 2.0: Fix holding mutex for to long

It is more efficient to release the mutex before notifying
corresponding conditional variable.

Test: N/A
Change-Id: I1a95efa51ea9b92dc31de5fdca7eb6ddfb0acba8
diff --git a/keystore2/src/database.rs b/keystore2/src/database.rs
index 0ce4ad6..9d20c75 100644
--- a/keystore2/src/database.rs
+++ b/keystore2/src/database.rs
@@ -150,6 +150,7 @@
     fn drop(&mut self) {
         let mut locked_keys = KEY_ID_LOCK.locked_keys.lock().unwrap();
         locked_keys.remove(&self.0);
+        drop(locked_keys);
         KEY_ID_LOCK.cond_var.notify_all();
     }
 }