Disable setting background color in QInputMethodEvent

IME managers may set background and foreground color of the composition
string. Since there is no Chromium API for setting the foreground color,
changing the background color to black may produce "black-on-black" text.
Chromium locks the background color to transparent thus we do the same
in this fix.

Task-number: QTBUG-58362
Task-number: QTBUG-60031
Change-Id: I939c8aaaa847de191890d6924ba3c7652465f8a0
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>