Fix arch in snapshot DepsMutator.

Suffixes attached to snapshot modules are based on the main architecture
(e.g. "arm64" for aosp_cf_x86_64_phone-userdebug), not the architecture
of arch variants.

Bug: 184588077
Bug: 184695066
Test: Run unit tests using:
Test: source build/envsetup.sh
Test: m -j nothing
Change-Id: Id8d002d45864a6fb12d642b60011e7a9eaf29e07