Split out osMutator from archMutator

Split the archMutator into two mutators, osMutator to create
android/linux/darwin variants, and archMutator to then further
split them into arm/arm64, etc.  When combined with alias
variants in Blueprint this will allow us to depend on the
"android" variant without having to know what architectures
were used to build it.

Test: m checkbuild
Change-Id: I0f51d60fc440cf5ae503904ce7849ec797af5fe2

android/module.go

@@ -417,6 +417,7 @@ type commonProperties struct {
 	} `android:"arch_variant"`
 
 	// Set by TargetMutator
+	CompileOS           OsType   `blueprint:"mutated"`
 	CompileTarget       Target   `blueprint:"mutated"`
 	CompileMultiTargets []Target `blueprint:"mutated"`
 	CompilePrimary      bool     `blueprint:"mutated"`
@@ -719,12 +720,6 @@ func (m *ModuleBase) visibility() []string {
 	}
 }
 
-func (m *ModuleBase) SetTarget(target Target, multiTargets []Target, primary bool) {
-	m.commonProperties.CompileTarget = target
-	m.commonProperties.CompileMultiTargets = multiTargets
-	m.commonProperties.CompilePrimary = primary
-}
-
 func (m *ModuleBase) Target() Target {
 	return m.commonProperties.CompileTarget
 }