IdleMaint: run IdleMaint if battery is charging

We've got reports where this ate battery too much during 3 to 4 AM.

Bug: 170441569
Bug: 168083554
Signed-off-by: Jaegeuk Kim <jaegeuk@google.com>
Change-Id: I8f41a8922034d3d625b90f70a523155d7f1c51e4
Merged-In: I8f41a8922034d3d625b90f70a523155d7f1c51e4