Add equals() and hashcode() to SpannableString

ActionBar uses a transition to animate text changes. This transition
depends on testing the equality of start/end text values in CharSequence
objects. Without equals(), SpannableString will return false for objects
whose references are different, but whose text is exactly the same.

This CL adds the equals() method, and the accompanying hashcode method,
to ensure that two Spanned implementations will always be equal
if their text and span data are equal.

Issue #10760075 Wrong unread count in actionbar

Change-Id: I5e77d40dd302eca035e8c56d40f3cd0aef8e6424