Support `enabled` flag in product variable config

Some Android.bp modules have `enabled: false` but only use a product
variable such as `source_build` to enable the module.  Currently b2build
does not handle this case at all. This commit adds the functionality
to support this use case.

Also, remove `__enabled` suffix in ProductVariable SelectKey.

Bug: 210546943
Test: go test ./bp2build
Topic: use_enabled_flag_product_variable_config
Change-Id: I459c17a84c172df010666391066bf4d11d19253e