Fix issue #5756204: Crespo IME briefly appears shortened when...

...rotating to landscape

When doing spell checking in the same process as the spell checker, we
need to make sure it is still done asynchronously.

Putting this in I noticed quite a few threading issues in this code, so
I also addressed those (which became very obviously a problem with the
async stuff here now).

Also tweaked the service side to run spell checking at background priority.

Change-Id: I01bafe3bec6bceeca911d6bf2f61a486a2fd4c48