Make clean_scratch_files work

clean_scratch_files.rc seems to never work as intended due to
missing domain transition rules.
Add a 'remount' domain to the platform policies to allow
remount-related operations, including clean_scratch_files.
Merge remount and clean_scratch_files binary to simplify the added
policies.

Bug: 204836146
Test: 1. Use a VAB device, for example bramble-userdebug
  2. adb remount system
  3. adb reboot fastboot && fastboot flash system system.img
  4. recovery (fastbootd) should mark the remount scratch as disabled
  5. fastboot reboot && adb shell mount | grep scratch => nothing
  6. adb logcat | grep 'clean_scratch_files|gsid' => shows that
    disabled image "scratch" is removed during boot
Change-Id: I18d543868d1f37d43b483eae7517b707e46df1bd
diff --git a/fs_mgr/Android.bp b/fs_mgr/Android.bp
index cb74ae0..5872dda 100644
--- a/fs_mgr/Android.bp
+++ b/fs_mgr/Android.bp
@@ -246,28 +246,12 @@
                 "-UALLOW_ADBD_DISABLE_VERITY",
                 "-DALLOW_ADBD_DISABLE_VERITY=1",
             ],
-        },
-    },
-    required: [
-        "clean_scratch_files",
-    ],
-}
-
-cc_binary {
-    name: "clean_scratch_files",
-    defaults: ["fs_mgr_defaults"],
-    shared_libs: [
-        "libbase",
-        "libfs_mgr_binder",
-    ],
-    srcs: [
-        "clean_scratch_files.cpp",
-    ],
-    product_variables: {
-        debuggable: {
             init_rc: [
                 "clean_scratch_files.rc",
             ],
         },
     },
+    symlinks: [
+        "clean_scratch_files",
+    ],
 }