Fix NPE when pasting non-text from Clipboard

When copying a link from a bookmark and then pasting it into
a textfield a NullPointerException will occur.
A ClipData.Item is not guaranteed to always contain a text string
and therefore getText() can be set to null.

Using method coerceToText() instead of getText() makes sure that
a text string is always returned.

Change-Id: I81343c0371835a3a7a52045dcd1760e69e59a967