Fix PersistableBundle handling in Parcel.writeValue

Previously, in Parcel.writeValue, PersistableBundle objects
would be handled as Parcelable types, since we check for
the Parcelable type before the PersistableBundle type (and
PersistableBundle implements the Parcelable interface).

Fix this by moving the PersistableBundle type "if" condition
above the Parcealble type "if" condition. Also, add a comment
that explicitly states this nuance, in order to prevent future
regressions.

BUG: 25815410
Change-Id: Ia86aa5fc39423422342df0609a0d37e7f38d7ddd