Prevent duplicate certificates in TrustedCertificateIndex

With the separate caching of intermediate certificates in
TrustManagerImpl a given intermediate may be passed into .index multiple
times. Avoid adding the certificate to the list each time.

(cherry-picked from commit d080e064abba665e2dde2a8c113f837ba4d9a85e)
Bug: 26232830
Change-Id: I6bed2c65d9e42e052b9b1b129200a997e7dca745
