Improve API signature.

There's nothing actionable for someone calling addToWhitelist() if the
addition fails (since it means the package wasn't found), so there's no
reason to have a return value. Removing the return value and clarifying
that calling the API will add the app to the permanent power save
whitelist.

Bug: 149216029
Test: atest PowerWhitelistTest
Change-Id: I54da34703c9f8862fa641f6b0313e6dfa37948e7