Remove InputMethodManagerDelegate#getLockObject()

This is a follow up CL to our previous CL [1], which introduced

  InputMethodManagerDelegate#getLockObject()

to clarify that "mServedView" and "mNextServedView" need to be guarded
with

  InputMethodManager#mH

as a short term workaround.

With this CL, above two fields are moved back to InputMethodManager
like they used to be before we introduce ImeFocusController [2].

This is a mostly mechanical refactoring.  Hopefully there should be no
observable behavior change.

 [1]: I9c072b829d1db1e68b65e766d764ee71cb16e6a2
      31e46784
 [2]: Ib455704fe1e9d243f93190a84f230210dbceac2a
      970d9d2e

Bug: 244504062
Test: presubmit
Change-Id: Ie3446e1d0f62c489195e31fc7f073020884d6cae