binding: navigation: add missing read lock around afb_event_push()

Add missing read lock around event pushing from value that isn't
required to be cached.

Bug-AGL: SPEC-2880
Change-Id: I7aae2ec9df722fdefcec2d35977251e008375d92
Signed-off-by: Matt Ranostay <matt.ranostay@konsulko.com>