Refactor ActivityStatus to not store current

- Refactor ActivityStatus to expose application level visibility over activity visbility.
- Add a listener for the visibility of the Application (are any Activities visible?)

BUG=341231
NOTRY=true

Review URL: https://codereview.chromium.org/159173002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252679 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252679