Keyboard dication input fixes

[LocalJoost]

Fixes two issues with the dictation of the keyboard.

1. The DictationInputManager never really stops recognizing. The DictiationRecognizer.Stop() fails with a "Assertion failed on expression: 'SUCCEEDED(hr)'" and internal state is not reset properly after that
2. If the user gazes at the keyboard while doing a dictation, the event speech event arrives twice.

Fixes:

1. Everything after the DictiationRecognizer.Stop is put into a separate coroutine. As a lot of stuff in the DictationInputManager is static, and there was some internal replumbing necessary to make it a) work and b) not break the interface.
2. In Keyboard.OnDictationResult now the eventData.used is checked. If true, input is ignored. If not, it is set to true after processing the input.