Unset default reco if selectableAsDefault=false.

A non selectableAsDefault recognition service can still become the
default if no other services are available.

Bug: 180964085
Test: non selectableAsDefault reco is unset at boot and replaced
Test: selectableAsDefault recos aren't affected
Test: system default reco still works
Change-Id: I429e6e457bc81ef0642ca90ccd9eb47eb10fbab8