Samsung's Korean keyboard has a bug where it always attempts to combine
characters based on its internal state, ignoring if and when the cursor
is moved programmatically. EG typing "ㄴㅇ" and then moving the cursor
back to the front of the text and typing "ㄴ" again would result in
"ㄴㅇㄴ", not "ㄴㄴㅇ".
Fully restarting the IMM works around this because it flushes the
keyboard's internal state and stops it from trying to incorrectly
combine characters. However this also has some negative performance
implications, so we only apply the workaround on Samsung devices set
to use Korean input.
This also effectively disables the feature on Samsung keyboards that
allowed users to re-open a composing region for previously typed
characters. See https://github.com/flutter/flutter/issues/29341#issuecomment-531283508.
Fixesflutter/flutter#29341.
Naively embedded platform views on Android were never able to receive
keyboard input, because they were never focusable. So far we've worked
around the limiation by hooking into InputMethodManager and proxying the
InputConnection from a focused window over to the embeded view.
Android Q changed InputMethodManager to be instanced per display instead
of a singleton. Because of this our proxy hook was never being called,
since it was being set up on a different instance of IMM than was being
used in the virtual display.
Update `SingleViewPresentation` to store the IMM from the focused window
and return it whenever there are any calls to `INPUT_METHOD_SERVICE`.
This hooks our proxy back into place for the embedded view in the
virtual display. This restores the functionality of our workaround from
previous versions.
Unfortunately there's still a lot of noisy error logs from IMM here. It
can tell that the IMM has a different displayId than what it's expecting
from the window.
This also updates the unit tests to support SDK=27. SDK 16 doesn't have
DisplayManager, so there were NPEs attempting to instantiate the class
under test.
