Ensure handwriting session can only be started from inside app bounds

Initialize the touchable region of the handwriting window to be cropped
to the focused window bounds. This means the handwriting window will
only spy over the app bounds.

When attempting to start a handwriting session, ensure that we are
currently recording a gesture in the handwriting window. This means we
can only start handwriting if the gesture originated within the app
bounds.

When a handwriting session starts, change the touchable region of the
handwriting window to be over the entire display, so that scribbles
outside the app window can sent to the IME.

Bug: 226081811
Test: manual: using test apps in split screen
Change-Id: I95b67ec7be40dc80ed67c91d0ee0b32296e532ca