Do lazy-unmount to /storage directly

From man 2 umount:
MNT_DETACH (since Linux 2.4.11)

Perform a lazy unmount: make the mount point unavailable for new
accesses, immediately disconnect the filesystem and all filesystems
mounted below it from each other and from the mount table, and
actually perform the unmount when the mount point ceases to be busy.

So we don't need to unmount the filesystems under it one by one.

Bug: 113796163
Test: atest android.appsecurity.cts.PermissionsHostTest#testInteractiveGrant23
Change-Id: I6a0422466a9865ff6d17122505ca73d041de9d54

Utils.cpp

@@ -755,29 +755,10 @@ bool IsRunningInEmulator() {
 }
 
 status_t UnmountTree(const std::string& prefix) {
-    FILE* fp = setmntent("/proc/mounts", "re");
-    if (fp == NULL) {
-        PLOG(ERROR) << "Failed to open /proc/mounts";
+    if (umount2(prefix.c_str(), MNT_DETACH)) {
+        PLOG(ERROR) << "Failed to unmount " << prefix;
         return -errno;
     }
-
-    // Some volumes can be stacked on each other, so force unmount in
-    // reverse order to give us the best chance of success.
-    std::list<std::string> toUnmount;
-    mntent* mentry;
-    while ((mentry = getmntent(fp)) != NULL) {
-        auto test = std::string(mentry->mnt_dir) + "/";
-        if (android::base::StartsWith(test, prefix)) {
-            toUnmount.push_front(test);
-        }
-    }
-    endmntent(fp);
-
-    for (const auto& path : toUnmount) {
-        if (umount2(path.c_str(), MNT_DETACH)) {
-            PLOG(ERROR) << "Failed to unmount " << path;
-        }
-    }
     return OK;
 }