Improve stopwatch landscape layout

Bug: 29074785
Bug: 29074834

- when ample space exists, stopwatch and laps list are centered
in each half of the landscape layout
- when limited space exists, laps list is granted its intrinsic width;
stopwatch consumes the remaining container width

Change-Id: I5b6e78d5b132730f6c862f0dbc896c1b958c44b7