Remove text spans from clipboard EditTextActivity display

Casts text from clipboard to a string before displaying it in the
clipboard text editor. Some apps use colored text spans to adjust
the color of text, which is retained in the clipboard. This will
cause us to show text in the wrong color (e.g. black-on-black); in
general we don't expect users to want to retain this metadata, so
just remove it for editing.

Bug: 278826009
Fix: 278826009
Test: manual (visual change)
Change-Id: I792f3116d3e0719874132ccb848262fa7c2d62af