java: delete package-export.apk before rebuilding

Delete package-export.apk before rebuilding to avoid aapt warnings
about using -f on existing packages.

Change-Id: I189486c4b3bd5fa46eff2e3b6a244d28329e7f02
diff --git a/java/app_builder.go b/java/app_builder.go
index 69c6fe1..96577bc 100644
--- a/java/app_builder.go
+++ b/java/app_builder.go
@@ -40,7 +40,7 @@
 
 	aaptCreateAssetsPackage = pctx.StaticRule("aaptCreateAssetsPackage",
 		blueprint.RuleParams{
-			Command:     `$aaptCmd package $aaptFlags -F $out`,
+			Command:     `rm -f $out && $aaptCmd package $aaptFlags -F $out`,
 			Description: "aapt export package $out",
 		},
 		"aaptFlags", "publicResourcesFile", "proguardOptionsFile", "javaDir", "javaFileList")