Make IInputMethodManager#reportActivityViewAsync() truly async

It turns out that IInputMethodManager#reportActivityView() [1], which
was added a few years ago to support CursorAnchorInfo API on edit
fields hosted on an ActivityView [1], is now also used by task
organizer [2].  This means that we would continue living with this
@hide API for a while even after ActivityView gets completely removed
at Bug 179161778.  Then, converting this IPC into truly asynchronous
would probably make sense.

There should be no behavior change unless the target IME client is
running on an embedded display environment.

 [1]: Id0411a80456182111bb5b681c6d1230b58e7ec2e
      b4f328a2
 [2]: Id422bb2547197c617f914ed7cf5085e02a1c3fb5
      a1f869d1

Fix: 186874025
Test: atest CtsWindowManagerDeviceTestCases:ActivityViewTest
Change-Id: I0463e8e9daaa8ab6d3eadb395843ca5e6bc1bb5e