Remove error from AndroidMkDataProvider.AndroidMk

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

Test: m -j checkbuild
Change-Id: I3e3b7251f32ffa84dbdfd0448faf248c306ca808
diff --git a/android/androidmk.go b/android/androidmk.go
index fd74507..2732c82 100644
--- a/android/androidmk.go
+++ b/android/androidmk.go
@@ -33,7 +33,7 @@
 }
 
 type AndroidMkDataProvider interface {
-	AndroidMk() (AndroidMkData, error)
+	AndroidMk() AndroidMkData
 	BaseModuleName() string
 }