Hook up major per-user call flows in IMMS

This is a kind of gigantic change that aims to hook up most of
remaining per-user call flows without breaking at least single-user
IME mode.

Most of changes are mechanical, including moving the following
@MultiUserUnawareField fields into UserData.

 * mBoundToMethod
 * mBoundToAccessibility
 * mImeBindingState
 * mCurClient
 * mInFullscreenMode
 * mCurInputConnection
 * mCurImeDispatcher
 * mCurRemoteAccessibilityInputConnection
 * mCurEditorInfo
 * mCurStatsToken
 * mEnabledSession
 * mEnabledAccessibilitySessions

So far all the end-to-end tests are passing and hopefully there is no
observable behavior change in the single user IME mode.

Bug: 305849394
Fix: 325515685
Fix: 349444857
Test: presubmit
Flag: EXEMPT refactor
Change-Id: I9d4615e6da979204c21555e040a687163c1460ab