Automatically set Androidmk suffix of snapshot

Androidmk_suffix has been captured with "m vendor-snapshot". But the
suffix is to avoid collision with core variants, so the suffix should be
based on the existence of core variants. This change sets the suffix by
using OtherModuleDependencyVariantExists. This way we can also avoid
mutators and global states.

Bug: 179665538
Test: m nothing (soong test)
Change-Id: I35343aa2c4c07782057b82e6dd0e705a786e55fc