Let IMM have internal APIs for layoutlib

In order to stub out InputMethodManager (IMM), currently layoutlib
tries to directly update internal details of InputMethodManager, which
is really fragile and hard to maintain.

With this CL, IMM officially defines an internal concept "EditMode"
and privately exposes internal APIs to layoutlib.

There should be no behavior difference in production build.

Bug: 115891476
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: Ide6aa5bc36c14449e87bec33a4962b87809b9d77