TextChange transition now uses CharSequence instead of String

TextChange made a bad assumption that target text views contained
String objects as their text, which is not always the case. For example,
Gmail ActionBar text views may contain SpannableString objects, which
caused a crash due to a bad cast operation.

Fix is to simply use CharSequence instead of String as the value that
is being stored and set on the text view.

Issue #10651858 TextChange transition casts CharSequence to String when not necessarily safe

Change-Id: I3f67f5d5e39201dc0fd2edcd3e4243033cf9ae08