ash: Improve low-battery notification thresholds.

Use rounded minute and percentage values when determining
when to show low-battery notifications. Also round
minutes-to-empty values shown in notifications.

BUG=chrome-os-partner:23458

Review URL: https://codereview.chromium.org/32443004

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@230353 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 230353