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
diff --git a/filesystem/filesystem_test.go b/filesystem/filesystem_test.go
index 8fe5dac..f767eae 100644
--- a/filesystem/filesystem_test.go
+++ b/filesystem/filesystem_test.go
@@ -670,7 +670,10 @@
 android_filesystem {
     name: "myfilesystem",
     deps: ["libfoo_has_no_stubs", "libfoo_has_stubs"],
-    linker_config_srcs: ["linker.config.json"],
+    linkerconfig: {
+	    gen_linker_config: true,
+	    linker_config_srcs: ["linker.config.json"],
+    },
     partition_type: "vendor",
 }
 cc_library {