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/cc/strip.go b/cc/strip.go
index 32ea38d..42c9137 100644
--- a/cc/strip.go
+++ b/cc/strip.go
@@ -52,7 +52,7 @@
 func (stripper *Stripper) NeedsStrip(actx android.ModuleContext) bool {
 	forceDisable := Bool(stripper.StripProperties.Strip.None)
 	// Strip is enabled by default for device variants.
-	defaultEnable := actx.Device()
+	defaultEnable := actx.Device() || actx.Config().StripByDefault()
 	forceEnable := Bool(stripper.StripProperties.Strip.All) ||
 		Bool(stripper.StripProperties.Strip.Keep_symbols) ||
 		Bool(stripper.StripProperties.Strip.Keep_symbols_and_debug_frame)