Docs: add a message to already-deprecated API

No change to logic. This adds a deprecation message to
`getSaveFormData()`. The API was already deprecated in Android O, but it
looks like we forgot a message. This copies the message from
`setSaveFormData()`, with small wording tweaks.

This adds some other javadoc formatting changes, like adding `@see`
links between the getter/setter methods and linking to
`Build.VERSION_CODES.O`.

Fixes: 310672520
Test: make offline-sdk-docs
Flag: DOCS_ONLY
Change-Id: Ia1648cc8fbb1360bdd0deef22e0eb481a29310d7