Use a boolean to determine if linker.config.pb should be generated

vendor and product generate a linker.config.pb even if
`PRODUCT_VENDOR_LINKER_CONFIG_FRAGMENTS` or
`PRODUCT_PRODUCT_LINKER_CONFIG_FRAGMENTS` are empty respectively. This
CL introduces a new property to determine if a filesystem should
generate a linker.config.pb. The autogenerated vendor and product
filesystem modules will set this property to true.

Test: go test ./filesystem
Bug: 376515221

Change-Id: Ibd007df0d287034f4d227a6054bd83937570ec27