Add REMOUNT_MODE_ANDROID_WRITABLE.

Constant to indicate a regular scoped storage view, with the sole
exception that Android/ should be writable.

Bug: 134706060
Bug: 146490513
Test: builds
Change-Id: Ifbcd4fd912ed4cc18d7c6e3eab2c582bc862e10c

binder/android/os/IVold.aidl

@@ -167,6 +167,7 @@ interface IVold {
     const int REMOUNT_MODE_INSTALLER = 5;
     const int REMOUNT_MODE_FULL = 6;
     const int REMOUNT_MODE_PASS_THROUGH = 7;
+    const int REMOUNT_MODE_ANDROID_WRITABLE = 8;
 
     const int VOLUME_STATE_UNMOUNTED = 0;
     const int VOLUME_STATE_CHECKING = 1;