Don't do a full reconnection, but just a stopIdling/startIdling pair. In
order to be able to do that, make sure the IDLE connection is fully shut
down when stopIdling() returns, for which some refactoring was needed to
avoid a deadlock on mIdledFolders: the ImapIdleListener callbacks
acquire this lock, so stopIdling() now MUST NOT be called with
mIdledFolders lock held.
Change-Id: Ifa1677d7845722ccee2b1b9380c7b7e4014bcd97
cb4fda8286