Add system_modules to droidstubs

This allows droidstubs to use the same system modules to create the
stubs that will be used to compile them. It improves consistency and
avoids droidstubs having to duplicate the libraries that make up the
system modules on its libs property.

Adds systemModules() to the sdkContext which allows consistent error
checking behavior between droidstubs and java_library.

Bug: 142534789
Test: m checkbuild
Change-Id: Ib2006906d9528a900f16851f50b62152ffb51a1b

java/sdk.go

@@ -39,6 +39,8 @@ var apiFingerprintPathKey = android.NewOnceKey("apiFingerprintPathKey")
 type sdkContext interface {
 	// sdkVersion returns the sdk_version property of the current module, or an empty string if it is not set.
 	sdkVersion() string
+	// systemModules returns the system_modules property of the current module, or an empty string if it is not set.
+	systemModules() string
 	// minSdkVersion returns the min_sdk_version property of the current module, or sdkVersion() if it is not set.
 	minSdkVersion() string
 	// targetSdkVersion returns the target_sdk_version property of the current module, or sdkVersion() if it is not set.
@@ -185,8 +187,18 @@ func decodeSdkDep(ctx android.BaseModuleContext, sdkContext sdkContext) sdkDep {
 			frameworkResModule: "framework-res",
 		}
 	case "none":
+		systemModules := sdkContext.systemModules()
+		if systemModules == "" {
+			ctx.PropertyErrorf("sdk_version",
+				`system_modules is required to be set to a non-empty value when sdk_version is "none", did you mean sdk_version: "core_platform"?`)
+		} else if systemModules == "none" {
+			// Normalize no system modules to an empty string.
+			systemModules = ""
+		}
+
 		return sdkDep{
 			noStandardLibs: true,
+			systemModules:  systemModules,
 		}
 	case "core_platform":
 		return sdkDep{