VNDK listing contains device modules only

Fix a bug where host-only modules were incorrectly listed as VNDK.
Also refactor VndkMutator() / apexVndkDepsMutator() module skipping
logic.

Bug: 158543482
Test: Add unit test to cc/cc_test.go
Change-Id: I50b09f526cbc081149d8241c2a091e3ee48ef4d7

android/module.go

@@ -830,6 +830,10 @@ func (m *ModuleBase) Host() bool {
 	return m.Os().Class == Host || m.Os().Class == HostCross
 }
 
+func (m *ModuleBase) Device() bool {
+	return m.Os().Class == Device
+}
+
 func (m *ModuleBase) Arch() Arch {
 	return m.Target().Arch
 }