Fix infinite recursion in hashcode of Spannables

An app created a SpannableStringBuilder, one of which's spans was the
instance of the string builder itself (that is, the builder contained a span
that was the builder). This caused infinite recursion in the hashcode()
method because it computes a hash from its fields, including all of its spans.

The fix detects the case where a span equals the current instance and
noops the computation on that span. A similar adjustment was made to equals()
to avoid the same recursion problem.

Issue #11051658 StackOverflowError in android.text.SpannableStringBuilder.hashCode

Change-Id: I742687ab32d81ac51c4b9135f698cf5e96a1d295