Add exported_plugins to java.Library.

The behaviour is similar to go/be#java_library.exported_plugins. Plugins added to exported_plugins of library X are not applied to the library itself, but rather to libraries that directly depend on the library X.

Test: m checkbuild
Bug: 139740873
Change-Id: I4042bd482ad9cb12d6fbaac51f039d38b1b7a428

java/aar.go

@@ -694,6 +694,10 @@ func (a *AARImport) ExportedSdkLibs() []string {
 	return nil
 }
 
+func (d *AARImport) ExportedPlugins() (android.Paths, []string) {
+	return nil, nil
+}
+
 func (a *AARImport) SrcJarArgs() ([]string, android.Paths) {
 	return nil, nil
 }