dragon: Add keyboard battery warning in DragonFirmwareKeyboardUpdater

Add the Pixel C keyboard battery warning notification in
DragonFirmwareKeyboardUpdater. The notification will show up when the keyboard
is connected and its battery level is low.

BUG=b/25369339
TEST=build, run the service and check the display

Change-Id: I117cd59987fe638c5ad77ae35d9731b5053764dd
4 files changed