Added option to check if an app is whitelisted for deviceidle.

Currently, there are only options to get the whole list:

  adb shell dumpsys deviceidle whitelist

Or to add/remove an app:

  adb shell dumpsys deviceidle whitelist +my.app
  adb shell dumpsys deviceidle whitelist -my.app

This change add an option to get the status of an app, so it can be used
on CTS tests:

  adb shell dumpsys deviceidle whitelist =my.app

BUG: 27127112
Change-Id: Ife4b8e2efa51e4ba5e1e2263e59884e5054f1e0a