[CB14.5] Stop copying WordComposers.

This is an overdue refactoring that I have been wanted to
do forever, and that we can at last do now that suggestions
come only from instances of BinaryDictionary instances.

With this, all unit tests and regression tests are
passing again.

Bug: 13667648
Bug: 13665487
Bug: 8636060
Change-Id: Ie23a19f9b7e692edf2ee0b107ddaff2d7db564b6