Add handleTrustStorageUpdate

This pruns all the stored trusted issuers so that changes to the system
or user CA store are detected. Currently this is only exposed as a
TestApi, but it can be hooked up to the trusted storage change event
in a future commit.

Bug: 27526668
Change-Id: Ic426254babab9a3177c968bc05b45e95eaac1fdd