Added phony module type for soong.

The "required" field within "phony" module in .bp file has to be
non-empty. And "phony" module doesn't contain any "src" files.
Bug: b/32981153
Test: make -j out/soong/build.ninja

Change-Id: Iaa2009467c44702572b97ca9cbd02c1d368c8d83

android/module.go

@@ -89,6 +89,8 @@ type ModuleContext interface {
 
 	Proprietary() bool
 	InstallInData() bool
+
+	RequiredModuleNames() []string
 }
 
 type Module interface {
@@ -784,6 +786,10 @@ func (ctx *androidModuleContext) ExpandSourcesSubDir(srcFiles, excludes []string
 	return expandedSrcFiles
 }
 
+func (ctx *androidModuleContext) RequiredModuleNames() []string {
+	return ctx.module.base().commonProperties.Required
+}
+
 func (ctx *androidModuleContext) Glob(globPattern string, excludes []string) Paths {
 	ret, err := ctx.GlobWithDeps(globPattern, excludes)
 	if err != nil {