Add proto2 files for client-side IME classes

This CL adds proto2 files which declare the structure of IME related
client states dumps. These files are compiled into individual classes
which contain a fieldId for every field to be saved in the
proto dump file.

Bug: 154348613
Test: make and flash to see compiled classes
Change-Id: Ie11fae6be4abeaf00871c382fda43b033c321721