Collect dist contributions from makeVarsInitProviders

Modules can export make vars not only using `MakeVars(...)`, but also
using `RegisterMakeVarsProvider(...)`. This change adds support for
soong only build to collect dist contributions added via custom
functions that are registed via `RegisterMakeVarsProvider(...)`.

Test: m droid dist --soong-only
Bug: 394365683
Change-Id: If5560ca32f2136ee41d24e2f6c08ad8ec954e5d6