Rename ModuleBase receivers

I3b23590b8ce7c8a1ea1139411d84a53163288da7 renamed AndroidModuleBase
to ModuleBase, but left the receivers called 'a'.  Rename them to
'm' to match the type.

Test: m checkbuild
Change-Id: I0ebb738152febfd9a2a56ac0b197f19f9e01a1e7
diff --git a/android/variable.go b/android/variable.go
index d039a16..b4f31c6 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -402,12 +402,12 @@
 	}
 }
 
-func (a *ModuleBase) setVariableProperties(ctx BottomUpMutatorContext,
+func (m *ModuleBase) setVariableProperties(ctx BottomUpMutatorContext,
 	prefix string, productVariablePropertyValue reflect.Value, variableValue interface{}) {
 
 	printfIntoProperties(ctx, prefix, productVariablePropertyValue, variableValue)
 
-	err := proptools.AppendMatchingProperties(a.generalProperties,
+	err := proptools.AppendMatchingProperties(m.generalProperties,
 		productVariablePropertyValue.Addr().Interface(), nil)
 	if err != nil {
 		if propertyErr, ok := err.(*proptools.ExtendPropertyError); ok {