Fix google-explicit-constructor warnings in keystore.

Declare explicit conversion constructors.

Bug: 28341362
Change-Id: Ibd0d6b7ec63ccb8f910e9da15771ff4d0031f4c4
Test: build with clang-tidy
diff --git a/keystore/auth_token_table.h b/keystore/auth_token_table.h
index a2f1446..bcf88fd 100644
--- a/keystore/auth_token_table.h
+++ b/keystore/auth_token_table.h
@@ -41,7 +41,7 @@
  */
 class AuthTokenTable {
   public:
-    AuthTokenTable(size_t max_entries = 32, time_t (*clock_function)() = clock_gettime_raw)
+    explicit AuthTokenTable(size_t max_entries = 32, time_t (*clock_function)() = clock_gettime_raw)
         : max_entries_(max_entries), clock_function_(clock_function) {}
 
     enum Error {