Fix issue #21813831: Need API for asking to be added to power whitelist

When launching the battery whitelist for a particular app, we now
default to the app being asked to enable adding to the whitelist, and
dismissing the dialog will dismiss the entire whitelist UI.

You can now launch the whitelist without specifying an app to just
get the regular UI.

Change-Id: Idf3840b8a30febe71fbd600969c257d72809643f