health VTS: Status::UNKNOWN is OK if not present

Documentation: https://source.android.com/devices/tech/power/batteryless

If a battery device is not detected, the following battery-related
defaults are used on Android 9 and higher:

* Present:              false
* Status:               unknown
* Remaining capacity:   0
* Health:               unknown
* AC charger online:    not modified

The previous version of the test failed devices if the vendor HAL
reported BatteryStatus::UNKNOWN. However, the tests were skipped if the
default HAL was the one being used, so this has not come up before for
other batteryless devices.

Bug: 142081126
Test: vts-tradefed run vts -m VtsHalHealthV2_0
Change-Id: I8ca758677478b47511e24990fee545fafa6c7f83