Move transition code to transition.go

Split the transition code out of mutator.go in preparation for adding
more complexity in TransitionMutator.

Bug: 372543712
Test: builds
Change-Id: I30f82aea1f296e0db365e0ca3606ef6a4c6cb778
diff --git a/android/Android.bp b/android/Android.bp
index dfea8f9..20cd28b 100644
--- a/android/Android.bp
+++ b/android/Android.bp
@@ -109,6 +109,7 @@
         "test_asserts.go",
         "test_suites.go",
         "testing.go",
+        "transition.go",
         "util.go",
         "variable.go",
         "vendor_api_levels.go",
@@ -154,6 +155,7 @@
         "singleton_module_test.go",
         "soong_config_modules_test.go",
         "test_suites_test.go",
+        "transition_test.go",
         "util_test.go",
         "variable_test.go",
         "vintf_fragment_test.go",