Introduce API to support Stylus Companion widget

Introduce APIs necessary for IMEs to be able to distinguish when to show
virtual keyboard vs companion widget based on toolType.

IME receives click toolType in:
1. IMS#onUpdateEditorToolType(int toolType) for every showSoftInput()
   called from IMM.
2. Initial value of toolType in EditorInfo#getInitialToolType().

Fix: 233108064
Bug: 210039666

Test: atest StylusHandwritingTest

Change-Id: I91ea9204ef62697b8942b0f350169382f18b3278