Sdk snapshot set compile_multilib per OsType

Previously, when an sdk snapshot only supported a single os type the
compile_multilib was set based on the multilib usages by the members
of that variant. After the change to support multiple os types per
snapshot the multilib setting was based on the multilib usages across
all the members of all sdk variants. That meant that if one os type
used only "64" and the other used "both" then they would both be
treated as "both" leading to missing variants when the snapshot was
unpacked.

This change tracks the multilib usages per os type and adds a property
for each one.

It intentionally changes a couple of tests:
1) Either by adding compile_multilib that is missing.
2) By targeting it at a specific os type instead of host.

The latter change is important to prevent the snapshot from being
used on a host platform (which will match the host target section)
but which is a different os type to the ones supported by the
snapshot.

Bug: 142935992
Test: m nothing
Change-Id: I883919b644292c3d019db223bb4fd5c11b39591f