Remove empty DepsMutator methods

Add an empty DepsMutator to ModuleBase so it doesn't have to be
implemented on every module that doesn't need it.

Test: all soong tests
Change-Id: I545a832a0dbf27386d3080377a75ea482cd9ce59
diff --git a/android/module.go b/android/module.go
index 551824d..f2f1af1 100644
--- a/android/module.go
+++ b/android/module.go
@@ -481,6 +481,8 @@
 	prefer32 func(ctx BaseModuleContext, base *ModuleBase, class OsClass) bool
 }
 
+func (a *ModuleBase) DepsMutator(BottomUpMutatorContext) {}
+
 func (a *ModuleBase) AddProperties(props ...interface{}) {
 	a.registerProps = append(a.registerProps, props...)
 }