Continue to @hide freeform decor caption APIs

Hiding the APIs for now since we're not releasing freeform yet and it's
better not to expose them now in case we'll decide to change them later.

May you make the P release!

Test: manual
Bug: 28774476
Change-Id: Ie4b4d1bdabef9b27d2eb1442ffc9f6c46ce7dfcb