Introduce a java_system_features_srcs wrapper for codegen

Use soong to aggregate all system feature-related build flags as input
to the existing system feature codegen tool. Use this instead of the
more clumsy raw genrule variant. This is an effective no-op in terms of
generated code or framework impact, but should improve maintainability.

Test: m --no-skip-soong-tests
Bug: 203143243
Change-Id: I67158dd6f0454556f8d75685cfc86a68b3e47089
diff --git a/android/config.go b/android/config.go
index 10bddf7..acb4a53 100644
--- a/android/config.go
+++ b/android/config.go
@@ -285,6 +285,10 @@
 	return c.config.productVariables.GetBuildFlagBool("RELEASE_CREATE_ACONFIG_STORAGE_FILE")
 }
 
+func (c Config) ReleaseUseSystemFeatureBuildFlags() bool {
+	return c.config.productVariables.GetBuildFlagBool("RELEASE_USE_SYSTEM_FEATURE_BUILD_FLAGS")
+}
+
 // A DeviceConfig object represents the configuration for a particular device
 // being built. For now there will only be one of these, but in the future there
 // may be multiple devices being built.