Encode default enabled state in OsType

Currently our only default-disabled Os is Windows since it's HostCross,
but we'll be adding non-default Host and Device types in the future.

Bug: 31559095
Test: out/soong/build.ninja is identical
Change-Id: I2bc3a3cc76f2c95ea040bc34ba6706fcc178c68d

android/module.go

@@ -355,7 +355,7 @@ func (a *ModuleBase) DeviceSupported() bool {
 
 func (a *ModuleBase) Enabled() bool {
 	if a.commonProperties.Enabled == nil {
-		return a.Os().Class != HostCross
+		return !a.Os().DefaultDisabled
 	}
 	return *a.commonProperties.Enabled
 }