fix(ui): extends predicates to mobile

[deven98]

Fixes #1796

Earlier implementation was only listening for keystrokes on desktop. New implementation also does the same by default but allows the user to customise and add their own send and clear predicates.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 14.28571% with 12 lines in your changes missing coverage. Please review.

Project coverage is 60.23%. Comparing base (1806fab) to head (591b563).

Files	Patch %	Lines
...er/lib/src/message_input/stream_message_input.dart	14.28%	12 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1947 +/- ## ========================================== - Coverage 60.26% 60.23% -0.04% ========================================== Files 317 317 Lines 18495 18506 +11 ========================================== + Hits 11146 11147 +1 - Misses 7349 7359 +10 ```

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