Fix mailbox finding and unit tests.

We seemed to not properly fallback to querying the database if the cache
did not contain a mailbox of a specific type. We may want to consider
caching mailboxes related to PIM data, but for now this is a quick fix.

Also fixes unit tests.

Bug: 5019661
Change-Id: Idcac0a6f15aa7f174890ae586f478fbb8f6e05b7
6 files changed