Add isEnabled API in SPP.

API isEnabled is used to specify whether the page is enabled during
migration. Once it is disabled, all its functionalities, such as
browsing / search / slice provider are disabled.

Bug: 244122804
Test: unit-test & local build gallery
Change-Id: I4f3f83e314b624fd2f6f83e0d1f64f426efb0350
9 files changed