Summary:
When we convert nested <Text> components to Spannable object we must enforce the order of spans somehow,
otherwise we will have Spannable object with unpredictable order of spans, which will produce unpredictalbe text layout.
We can do it only using `Spannable.SPAN_PRIORITY` feature because Spannable objects do not maintain the order of spans internally.
We also have to fix this to implement autoexpandable <TextInput>.
Reviewed By: achen1
Differential Revision: D5811172
fbshipit-source-id: 5bc68b869e58aba27d6986581af9fe3343d116a7