split apex.go

apex.go is too big. Separate the build rule and android.mk generation
logic into builder.go and androidmk.go, respectively. prebuilt_apex is
moved to prebuilt.go as well.

No refactoring has been made other than the splitting.

Test: m
Change-Id: I839ab0a1ba2b70ce82d98ac1fa8e3534808b5fd3
diff --git a/Android.bp b/Android.bp
index c73c3da..7259255 100644
--- a/Android.bp
+++ b/Android.bp
@@ -458,8 +458,12 @@
         "soong-python",
     ],
     srcs: [
+        "apex/androidmk.go",
         "apex/apex.go",
+        "apex/builder.go",
         "apex/key.go",
+        "apex/prebuilt.go",
+        "apex/vndk.go",
     ],
     testSrcs: [
         "apex/apex_test.go",