Unhide RecoverySession#recoverKeyChainSnapshot() method

The other methods expose raw key materials, which is a security flaw. This
new API is already being used by GMSCore, via reflection (although falling
back to the old methods if it is not available). Would be good to switch it
on ASAP.

Bug: 74345822
Test: Tested with GMSCore
Change-Id: I30d53c9e825888d1122c72d23b7c1c10c6edb1e9