Prevent poorly-extended ParcelableSpans from writing to parcels

If a developer extends an existing ParcelableSpan and overwrites
writeToParcel, TextUtils will crash when attempting to unparcel
since the span type ID is not accurate. This CL makes a separate
framework-private method for writeToParcel to ensure that even if
a developer extends a ParcelableSpan class, they won't modify the
parceling or unparceling code that's tied to the span type ID.

Bug: 21274544
Change-Id: If4c3506a55999df7a3b6d87cb8d550235d7a02c6