Generate java_api_contribution module from droidstubs module

Context
- droidstubs module are either generated from the java_sdk_library
  module or defined in the bp files.
- Since droidstubs module contains API text file property,
  java_api_contribution module can by dynamically created from
  droidstubs.

Implementation
- Add `api_surface` property in droidstubs module. This property is
  either inherited from the java_sdk_library or written in the module
  definition in the bp file.
- Add defaultable hook in droidstubs module to generate the child
  java_api_contribution module.

Test: m
Change-Id: Ica43d65614723c623cd0c155266f9844e69e5d5e

java/java_test.go

@@ -1840,6 +1840,20 @@ func TestDeviceBinaryWrapperGeneration(t *testing.T) {
 		}`)
 }
 
+func TestJavaApiContributionEmptyApiFile(t *testing.T) {
+	testJavaError(t,
+		"Error: foo has an empty api file.",
+		`java_api_contribution {
+			name: "foo",
+		}
+		java_api_library {
+			name: "bar",
+			api_surface: "public",
+			api_contributions: ["foo"],
+		}
+	`)
+}
+
 func TestJavaApiLibraryAndProviderLink(t *testing.T) {
 	provider_bp_a := `
 	java_api_contribution {