Remove text spans from clipboard UI

Some apps set the text color by using ForegroundColorSpans. When
that text is copied, we currently preserve any existing text spans
in the clipboard UI. In the specific case of a dark mode app (using
light-colored text), this causes a visible problem because our UI's
background color is always light even in dark mode.

This change casts input text to a string before setting it in the
TextView, effectively removing spans.

Bug: 275607972
Fix: 275607972
Test: manual (visual change only)
Change-Id: Ib9feb19b99aef1c35e404bd02aaf3e0baf00d284