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This CL adds BluetoothBatteryClient which is a proxy to BlueZ's
org.bluez.Battery1 interface. Future CLs will interact with this new
interface to get battery level of Bluetooth devices and store the
value to BluetoothDevice object.

Battery API Doc:
https://chromium.googlesource.com/chromiumos/third_party/bluez/+/refs/heads/chromeos-5.54/doc/battery-api.txt

Bug: b:160905767
Change-Id: I2bee100b25518b48a296b87657026769058384d3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2296266
Commit-Queue: Sonny Sasaka <sonnysasaka@chromium.org>
Reviewed-by: Ryan Hansberry <hansberry@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789676}