Fix layoutlib tests.

The date picker and calendar widgets show the current date. Comparing
widgets showing current date with a static golden image wasn't the best
idea.

- Change the widgets to custom widgets that set a predefined date/time
  so that the rendering is independent of the system date.
- Change the layout slightly to make it look nicer.
- Update gradle plugin version.

Change-Id: Ia3fb4965a372a28087a0d8c9df40fd6779df6050