When an AndroidView gains focus we invoke the(newly introduced)
'TextInput.setPlatformViewClient' text_input system channel method
which sets the platform view as the text input target.

When the AndroidView loses focus we send a clearFocus message to
platform views system channel(so the engine can clear the focus from
the platform view).

This PR is going to land before the engine implementation is rolled to
the framework, we swallow MissingPluginException for the newly
introduced method channel methods so this is a no-op before the engine
is ready(after the engine is rolled with the corresponding change I'll
remove the logic to swallow the exceptions).

The engine counterpart is in: flutter/engine#9203