recovery: Extend A/B downgrades for older SPL

Some OEM devices lack the ability to sideload updates through stock
recovery, which can prevent users from reverting to older builds.
This change adds support for downgrading the security patch level
(SPL) on A/B devices, gated by a property and user confirmation.

Since SPL downgrades may trigger anti-rollback protections on certain
devices, recovery now warns the user before proceeding.

Change-Id: If8a3ab7347e51c90337c4044826226ea11d6a327