Assume UNSUPPORTED if the battery capacity level sysfs node does

not exist.

Bug: 163382088
Test: Ensure that, if the ./capacity_level does not exist,
  that the device will shut down at 0 % SOC in an Health 2.1
  implementation
Change-Id: I00822b4bfba5d823c1a30163ea6824e59d3aae32