Rename the title of the first gird option

Rename it from "Default" to "5x5" to avoid misleading
when Launcher's default grid configuration is not 5x5.

Screenshots:
  Before: https://screenshot.googleplex.com/4aTmiqZ3RUFbj3t.png
  After: https://screenshot.googleplex.com/6ecXhDa9mDUi9aC.png

Bug: 191118996
Test: manual
Change-Id: Ic2f0cfaf7d783b489123af7426d26742afd915c8