remount: Use /data for backing scratch storage.

Currently, this is only enabled for Virtual A/B devices. When /data uses
F2FS, then proper pinning support must be enabled.

Because /data cannot be mounted in recovery, we can't delete the scratch
image backed by /data while in fastbootd. Instead, we mark it as
disabled in /metadata. The remount command now has an init script that
checks for and removes a disabled scratch partition.

Bug: 134949511
Test: adb remount on V A/B device with patched F2FS
Change-Id: Ifc8720378259654472d3822e97059b6c366f601d
diff --git a/fs_mgr/Android.bp b/fs_mgr/Android.bp
index 34c64d2..7a88aa3 100644
--- a/fs_mgr/Android.bp
+++ b/fs_mgr/Android.bp
@@ -91,6 +91,7 @@
     },
     header_libs: [
         "libfiemap_headers",
+        "libstorage_literals_headers",
     ],
     export_header_lib_headers: [
         "libfiemap_headers",
@@ -165,7 +166,7 @@
         "libcrypto",
         "libext4_utils",
         "libfec",
-        "libfs_mgr",
+        "libfs_mgr_binder",
         "liblog",
         "liblp",
         "libselinux",
@@ -187,4 +188,26 @@
             ],
         },
     },
+    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",
+            ],
+        },
+    },
 }