Set compile_multilib for SDK snapshots for device.

Previously compile_multilib was ignored for device and defaulted to
"both", which could lead to an internal inconsistency in the snapshot
if a member was 32- or 64-only.

This also implements common property optimisation for compile_multilib,
by adding a struct that can be extended to other properties in the
future.

Test: m nothing
Bug: 151303681
Change-Id: I3e96222858e5e755173aca03508a23c431977e2a