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Also implements getLastWrittenValue() and converts two tests to use
these.

FakeRemoteGattDescriptor::setNextWriteResponse allows tests to set the
next response for a write.

BUG=719825

Change-Id: I2807d23a55f79eeca35338e25916c6d5e0115e89
Reviewed-on: https://chromium-review.googlesource.com/964901
Commit-Queue: Ovidio Henriquez <odejesush@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Giovanni Ortuño Urquidi <ortuno@chromium.org>
Reviewed-by: Conley Owens <cco3@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545508}