rust modules respects the installable property

So far, the property wasn't respected.

Bug: N/A
Test: m
Change-Id: Ie3b011250595f02c3ab315efbac6694df3e181e7

rust/rust.go

@@ -663,7 +663,7 @@ func (mod *Module) CoverageFiles() android.Paths {
 }
 
 func (mod *Module) installable(apexInfo android.ApexInfo) bool {
-	if !mod.EverInstallable() {
+	if !proptools.BoolDefault(mod.Installable(), mod.EverInstallable()) {
 		return false
 	}