Notify new IME target before IME starts interacting

Previously, there was a time window between when an IME starts new
input and when the IME issues an IPC IMM#setImeWindowStatus() so that
WindowManagerService (WMS) can be notified about the new IME target
window.

With this CL, it is now guaranteed that WindowManagerService (WMS) is
always notified about the new IME target window before IME starts
interacting with that window.

Note that WMS is not using notified IME target window yet hence there
should be no user-visible behavior change.

Bug: 110531072
Test: atest CtsInputMethodTestCases CtsInputMethodServiceHostTestCases
Change-Id: I032b91ce722a16b17518a5d88015c574d7d3e51b