Address NewApi warning in device/battery.

This moves the Lollipop-specific code in BatteryStatusManager into a
separate method and annotates it with @TargetApi(LOLLIPOP) so appease
Android lint.

BUG=411461

Review URL: https://codereview.chromium.org/975963003

Cr-Commit-Position: refs/heads/master@{#319115}