Touch keyboard keydown handling inserted ordinary text directly and
canceled the browser edit. On iOS this prevented the hidden textarea
from maintaining its autocorrect context. Let unmodified editing keys
reach the browser's input path while keeping modified shortcuts in GPUI.

The hidden textarea's focus and blur were also treated as window
activity. Dismissing the keyboard could mark a visible page inactive and
throttle its frames. Derive activity from browser-window focus and
document visibility instead, including after an IME focus cycle. Avoid
canceling touchend when the browser marks it noncancelable during
scrolling.

Release Notes:
- N/A
- [GPUI] N/A