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This switches the IME and Assistant services to use ServiceBinding
instead of deprecated ServiceContext & EmbeddedService APIs. Also moves
their tests off of the deprecated ServiceTest framework and changes them
to be simple unit tests.

TBR=dcheng@chromium.org

Bug: 891780,906239
Change-Id: I047fd3879da1c4eb89643717cd3d247cf638ef9b
Reviewed-on: https://chromium-review.googlesource.com/c/1343037Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Leo Zhang <googleo@chromium.org>
Reviewed-by: Tao Wu <wutao@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Commit-Queue: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#609812}