Introduce a sequence number to avoid redundant IPCs

This CL introduces a sequence number to avoid redundant IPCs
between IMM and IMMS. The basic concept is that:
1. IMMS maintains a sequence number for #notifyUserAction.
2. IMMS increases the sequence number whenever the current
   IME/subtype is changed.
3. IMMS notifies the new sequence number to IMM.
4. IMM maintains the last sent sequence number and the next
   sequence number. IMM should call #notifyUserAction only
   once per particular sequence number.
5. IMMS ignores #notifyUserAction if the specified sequence
   number is not the expected one.

BUG: 7043015
Change-Id: I19ad8542659bc092b92ee13eb9f1d68ddd4b815a