Add some friendly error messages for java_sdk_library.

The stubs will always fail to build without at least these
three arguments. Give a clear error message when this happens
instead of failing further into the build.

Bug: 77575606
Test: make <x>.stubs for sdk_library "x" without these args
Change-Id: If3142ecc73d6428f7e3120788f8de64d56a777ad

java/sdk_library.go

@@ -485,6 +485,13 @@ func javaSdkLibraries(config android.Config) *[]string {
 // once for public API level and once for system API level
 func sdkLibraryMutator(mctx android.TopDownMutatorContext) {
 	if module, ok := mctx.Module().(*sdkLibrary); ok {
+		if module.properties.Srcs == nil {
+			mctx.PropertyErrorf("srcs", "java_sdk_library must specify srcs")
+		}
+		if module.properties.Api_packages == nil {
+			mctx.PropertyErrorf("api_packages", "java_sdk_library must specify api_packages")
+		}
+
 		// for public API stubs
 		module.createStubsLibrary(mctx, apiScopePublic)
 		module.createDocs(mctx, apiScopePublic)