BackgroundDexOptService: skip low battery check if not present

If no battery is present, assume no need to check for low battery.

Do not interpret battery level values if battery is not present.

Bug: 34507420
Test: manual (temporary log messages with battery forced not present)
Change-Id: Iebc64bc890808d583d0bd95a31f17b6363b7ba40
Merged-In: Iebc64bc890808d583d0bd95a31f17b6363b7ba40