Trebuchet : Use consistent shared_prefs

PreferenceFragments by default use getDefaultSharedPreferences(), which
is inconsistent with the rest of our settings.

This patch ensures its consistent and renames SETTINGS_KEY away from
package name space to avoid future confusion.

Change-Id: Ic6323a6b392e01b27f7ba20cbd61ec9705bc5845
Trebuchet : Layout fixes

- Fixes excessive padding of searchbar on tablets
- Ensures proper use of numCols regardless of screen width

Change-Id: I42215db1ae328be16b0d996c26a25431442bcf97