Allow customizing stripping behavior with product variable

aosp/3530072 made the default for host modules be to not strip them,
but that increases the size of our build tools prebuilts. Add a flag
that can be enabled on the build-tools build.

Test: Manually
Change-Id: I48173a52fe1a7068731ce0aca2367a0ebee0d068
diff --git a/android/variable.go b/android/variable.go
index 853d8ef..9905b77 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -554,6 +554,8 @@
 	OdmManifestFiles       []string `json:",omitempty"`
 
 	UseSoongNoticeXML *bool `json:",omitempty"`
+
+	StripByDefault *bool `json:",omitempty"`
 }
 
 type PartitionQualifiedVariablesType struct {