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This CL adds a new method, EnableIme(). ArcInputMethodManagerService
observes changes of the list of the active input methods and
enables/disables IMEs in container side if any ARC IME is
enabled/disabled.
This CL also introduces new category of input method id, ArcImeId.
It starts with '_arc_ime_' and used for distinguish Arc IMEs from other
extension-based IMEs.

Bug: 845079
Test: Unit tests are added.
Change-Id: I94c673c96e4157d8c9f18896606144f91438b7dc
Reviewed-on: https://chromium-review.googlesource.com/1098869
Commit-Queue: Yuichiro Hanada <yhanada@chromium.org>
Reviewed-by: Yusuke Sato <yusukes@chromium.org>
Reviewed-by: Shu Chen <shuchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#567563}