USB drives mount but they are not visible to apps

[fcatrin]

When inserting an USB drive, it is shown in the Storage settings and it is listed in the mounted filesystems (mount and/or df commands), but it is not accessible to the apps because:

a) Non-system apps use the READ/WRITE_EXTERNAL_STORAGE permission that is not present in the system permissions (platform.xml)
b) The drive is not mounted as public in /storage because MountService doesn't flag it as Visible, only Primary Public or Adpotable drives are marked as Visible.

Take this (ugly) patch as a workaround to make USB drives always visible to apps and to include the needed permissions.

--- frameworks/base/data/etc/platform.xml.original	2017-07-03 01:04:10.236623976 -0400
+++ frameworks/base/data/etc/platform.xml	2017-07-03 01:05:29.066369954 -0400
@@ -59,6 +59,12 @@
         <group gid="log" />
     </permission>
 
+    <permission name="android.permission.WRITE_EXTERNAL_STORAGE" >
+        <group gid="media_rw" />
+        <group gid="sdcard_r" />
+        <group gid="sdcard_rw" />
+    </permission>
+
     <permission name="android.permission.WRITE_MEDIA_STORAGE" >
         <group gid="media_rw" />
         <group gid="sdcard_rw" />
--- ./frameworks/base/services/core/java/com/android/server/MountService.java.original	2017-07-02 02:15:51.744912102 -0400
+++ ./frameworks/base/services/core/java/com/android/server/MountService.java	2017-07-02 02:16:33.455624350 -0400
@@ -1185,7 +1185,7 @@
 
             // Adoptable public disks are visible to apps, since they meet
             // public API requirement of being in a stable location.
-            if (vol.disk.isAdoptable()) {
+            if (true || vol.disk.isAdoptable()) {
                 vol.mountFlags |= VolumeInfo.MOUNT_FLAG_VISIBLE;
             }