Add CommonOS variant for sdk

Adds a CommonOS variant for sdk that depends on the os specific
variants and is used to generate a single sdk for multiple OsTypes,
e.g. host linux and android.

At the minute the member types only support a single OsType but the
basic mechanism for managing the CommonOS variant and collating the
variants across all of them is there.

The only visible effect of this change is that the location of the
generated snapshot is changed, it is no longer os specific and instead
is in the same location irrespective of which os it is built for.

A lot of tests needed to be changed to specify "common_os" as the
variant type instead of the specific os type. As that is the same across
all tests it is hard coded in CheckSnapshot method.

Test: m nothing
Bug: 150451422
Change-Id: If36be39b06d6910453649f7c288c2d34f688b2f4