Convert arch and os mutators to TransitionMutators

Replace archMutator and osMutator with TransitionMutators.

Bug: 319288033
Flag: EXEMPT refactor
Test: all soong tests pass
Test: no change to out/soong/*.ninja
Change-Id: I92a4d7c895dd79f1dd8064d0ca90e7010b563525

android/arch_test.go

@@ -451,7 +451,7 @@ func TestArchMutator(t *testing.T) {
 
 	for _, tt := range testCases {
 		t.Run(tt.name, func(t *testing.T) {
-			if tt.goOS != runtime.GOOS {
+			if tt.goOS != "" && tt.goOS != runtime.GOOS {
 				t.Skipf("requries runtime.GOOS %s", tt.goOS)
 			}