Deprecate MissingMethodFlags to preserve invocation order

This CL effectively replaces my previous CL [1], which made
unimplemented methods in InputConnection not fatal errors, with a
simplified implementation that still gracefully take care of
unimplemented methods without causing app crashes.

Instead of propagating missing method information from the IME client
to the IME process, this CL will simply catch AbstractMethodError in
the IME client process.  Doing so enables us to

 1. preserve the strict invocation order of InputConnection APIs, and
 2. achieve the same goal with fewer lines of code.

The additional cost of throwing (and catching) AbstractMethodError
every time the IME calls an unimplemented InputConnection API can be
justified as it is really an exceptional scenario, and avoiding it
would require extra maintenance cost as seen in
InputConnectionInspector.

The above overhead (and complexity) due to AbstractMethodError can be
avoided by adding default implementations to those InputConnection
APIs, but doing so requires API signature update hence API council
approval to go ahead, which is to be discussed in Bug 199934664.

 [1]: I3c58fadd924fad72cb984f0c23d3099fd0295c64
      19a80a1e

Bug: 27407234
Bug: 27642734
Bug: 27650039
Bug: 194110780
Test: atest CtsInputMethodTestCases
Change-Id: I9e801e92496a6e16cee37664870c97ed096f1413