apex_sets is added to apexkeys.txt

apex_sets is a new module type that can be used to deliver pre-signed
APEXes, which previously could be done only via prebuilt_apex.

Soon gnow understands apex_sets module types and emits the signing info
of the modules to apexkeys.txt

Bug: 158729168
Test: m
Change-Id: I9507375342ec053309660d94c931a79bf4f21218