Extract common measurement in widgets pickers to its base class

Bug: 191644950
Test: 1. Open the full widget picker. Observe the clock widgets'
         previews are shown properly. Then, rotate the screen and
         observe the clock widgets' previews are shown properly.
      2. Open the bottom widgets picker for clock. Observe the
         clock widgets' previews are shown properly. Then, rotate
         the screen and observe the clock widgets' previews are shown
         properly.
      3. Repeat 1, 2 after changing the system navigation from button
         to gesture.
Change-Id: I564fc2ce0baf3103ae77499380ad69ec38ac6930