Use getLockObject to sync ImeFocusController#{mServedView, mNextServedView}

As CL[1] added InputMethodManagerDelegate#getLockObject that used for
protecting mServedView/mNextServedView in ImeFocusController when we
exposed getServedView/getNextServedView for InputMethodManager to access
with thread safty.

Add synchornize block for all places that accessing
{mServedView, mNextServedView} in ImeFocusController.

[1]: I9c072b829d1db1e68b65e766d764ee71cb16e6a2

Bug: 244504062
Test: atest CtsInputMethodTestCases
Change-Id: I8357bfafeaf0a40e3b74eec2fc59a47309451f18