Fixes the bug around misbehavior with empty homepage setting

  Bug: 2911567
  Setting the homepage to "" resulted in odd behavior. This changes
  it so that entering "" fails with an invalid URL error (now a toast
  instead of a dialog). The "Use Current" button was replaced with a
  "Set to..." button that allows the user to specify whether they want
  to set the homepage to the current page, a blank page, or the default page.

Change-Id: I2d0cac158f89ace6ecab6165a3768419ba3252d4
3 files changed