[Keyboard][Handwriting] Replace HandwritingPadsManager with HandwritingP...

...adTargetHandler
Update the comment on line 16:

    The caller of ActiveTargetsManager can set the following properties to alter the behavior:
    * blockNewUserPress: when set to true, any new presses are not handled.
    * blockTargetMovedOut: when set to true, presses won't be able to move out of the current target.