aflags: Ensure <partition>/aconfig_flags.pb exists before reading

In some build targets/combinations aconfig_flags.pb
may not be generated.

For example in GSI only system/aconfig_flags.pb exists
and system-only updates won't have vendor/aconfig_flags.pb.

Test: m, aflags
Change-Id: I136345a8748bc6d80bd98e673f90e0a2ab380ab1