Remove inaccurate words "main thread" from IInputConnectionWrapper

This is a clean-up CL for up my CL [1], which introduced

  InputConnection#getHandler()

per request from the Chromium team.

This CL only renames misleading and/or inaccurate code commends and
field names.  There should be no observable behavior change.

Even before my change [1], IInputConnectionWrapper had been
responsible for re-dispatching incoming IPCs onto the "UI thread"
obtained from View#getHandler(), which is not guaranteed to be the
"main thread" in some rare situations.

With my change [1], the target thread is no longer limited to the UI
thread.

This CL removes misleading and confusing "main" terminology from the
variable names and comments for future readers.

 [1]: Id9e579bb3e2966986cdcb1c34bc8cacfeca2e1a9
      612cce92

Bug: 26945674
Bug: 192412909
Test: presubmit
Change-Id: Ibb31da4f66e8a6cd35f93c3ca1cc0f871dfb3b73