Remove error from AndroidMkDataProvider.AndroidMk

It's never anything except nil, and it unnecessarily complicates
the implementations.

Test: m -j checkbuild
Change-Id: I3e3b7251f32ffa84dbdfd0448faf248c306ca808

android/androidmk.go

@@ -33,7 +33,7 @@ func init() {
 }
 
 type AndroidMkDataProvider interface {
-	AndroidMk() (AndroidMkData, error)
+	AndroidMk() AndroidMkData
 	BaseModuleName() string
 }