utils/ServiceCache: Make isValid public

Some of our apps may want to know if the ServiceCache was determined to
be valid prior to using it, so that they can retry their request in case
the cache was stale.

Change-Id: I106f11a18cd2290df9c18f29a8e779aacb5ef552
Signed-off-by: William A. Kennington III <wak@google.com>
1 file changed