Fix Bluetooth enable dialog to match Android style guide.

When an app requests to enable Bluetooth and/or Bluetooth discovery,
we show a dialog for user confirmation. Remove the dialog title,
update the message text and button labels to be more descriptive,
and use the standard dialog layout instead of a custom layout.

Also fixes the button layout on the Bluetooth permission test app so
that the "Discoverable" button doesn't wrap to two lines.

Bug: 6001468
Change-Id: I731e2f31b4c822395fc3f83584a092550d9ae7d3