Let A11yIME use its own IPC definitions

This is a follow up CL to our previuos CL [1], which enabled
AccessibilityService to use a subset of InputConnection APIs.

In that CL we have reused existing AIDL interfaces that were designed
and maintained for IMEs for simplicity, where a non trivial amount of
unnecessary IPC endpoints were included.

From the security and maintainability viewpoints, however, exposing
unnecessary IPC endpoints is discouraged in general.  To address such
concerns this CL introduces a set of dedicated IPC definitions for
A11yIME so that we do not need to reuse IPCs for IMEs.

This CL also stops passing InputBinding object to A11yIME process as
it contains IInputContext Binder Proxy, which can still be used to
directly invoke fallback InputConnection.  This is doable now because
A11yIME no longer relies on fallback InputConnection [2].

This CL is should not have any observable changes in the semantics.
End-to-end CTS tests guarantee that everything is still working as
intended now and in the future.

 [1]: Ia651a811093a939d00c081be1961e24ed3ad0356
      fb17e5ae
 [2]: I2af3cd50444d8ddf25aa0f6479238156914e6fff
      dc635efb

Fix: 215633021
Fix: 215636776
Test: atest CtsInputMethodTestCases:AccessibilityInputMethodTest
Test: atest CtsAccessibilityServiceTestCases:AccessibilityInputConnectionTest
Test: atest CtsAccessibilityServiceTestCases:AccessibilityImeTest
Change-Id: I5ff2e804cbcf90828370a0612ff54111130bdff4