This support enables specifying properties of the type "map" within a
Soong module, but explicitly does not allow them to be used within a bp
file.
This means that rather than specifying each arch/os/target within a
struct to support arch-variant properties/attributes, we can use a map.
This allows us to simplify the implementation of LabelAttribute,
StringListAttribute, and LabelListAttribute as the number of select
statements supported becoming large and hard results in a lot of
duplication.
Test: go test blueprint tests
Test: m nothing
Change-Id: I88cc5952a6bdb60a2344fa0737216f016086cea5
c1ccfee2bd