Make InputMethodManagerService#mMethodMap private

As a preparation to support IMEs for concurrent multi users, this CL
makes

  InputMethodManagerService#mMethodMap

as a private field to simplify inter-class data dependencies.

This is a mechanical refactoring CL. There must be no observable
behavior change.

Fix: 30987034
Test: presubmit
Change-Id: Id08aa0560e4bee6f837fa14f40d2d234442c7820