Editor: Account for a split cursor in the content rect calculation

In some cases, the cursor in a TextView is split. This happens with
RTL languages. For this case we need to ensure neither of the half
cursors is obscured by the Floating Toolbar, hence we take the
minimal rectangle that contains both cursors as the content rect.

Change-Id: I2ce411793fcba2140c4ad7fb7caaf03593620484