Create a new MOUNT_EXTERNAL_LEGACY storage mode.

Apps that are already installed on the device before isolated_storage
feature is enabled will be granted MOUNT_EXTERNAL_LEGACY mode. In this
mode, /mnt/runtime/write will be mounted at /storage giving them same
level of access as in P.

A new mount directory /mnt/runtime/full is also created which will be
used for mounting at /storage for apps started with MOUNT_EXTERNAL_FULL
mode. This will allow apps with WRITE_MEDIA_STORAGE permission to
read/write anywhere on the secondary devices without needing to bypass
sdcardfs.

Bug: 121277410
Test: manual
Test: atest android.appsecurity.cts.ExternalStorageHostTest
Change-Id: I4ec73276d7c586ae4afc482580d1eb8ee03d5be1