ash: Move peripheral battery notifier to ash/.

Test: ash_unittests PeripheralBatteryNotifierTest.*
Bug: 761556
Change-Id: I45f691d17380812c37de13d5d3a8df9d87ad2fa6
Reviewed-on: https://chromium-review.googlesource.com/656513


Reviewed-by: Max Moroz <mmoroz@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Sammie Quon <sammiequon@chromium.org>
Cr-Commit-Position: refs/heads/master@{#501354}