RotarySelector widget: add callback for "grabbed" state changes.

This is the frameworks/base part of the fix for Bug 2158434: add a new
callback to the OnDialTriggerListener interface, so the RotarySelector can
tell the app about state changes between NOTHING_GRABBED and
LEFT_HANDLE_GRABBED and RIGHT_HANDLE_GRABBED.

BUG=2158434
DRNO=timsullivan

TESTED=regular incoming calls, call waiting calls, answering a call,
       rejecting a call.  Also verified I didn't affect the
       lock screen at all.