storageproxyd: Call open_possibly_mapped_file on all open paths

Remove the direct open paths from storage_file_open and always call
open_possibly_mapped_file instead.

This fixes two bugs.

If /data became available between trying to open the symlink and
checking the root path, the open would succeed on the backing_storage
but storage_file_write would fail because the it would try to create the
symlink again.

If storageproxyd restarts when the symlink does not exist, but the
storage_server thinks the file it opened with STORAGE_FILE_OPEN_CREATE
before the restart should still exist, it might try to re-open it
without STORAGE_FILE_OPEN_CREATE. This would fail because
storage_file_open would not call open_possibly_mapped_file unless
STORAGE_FILE_OPEN_CREATE was passed.

Test: --test com.android.storage-unittest.td-init-check-clean
Change-Id: I2e32340f09381465a82812ad8963a9e76ea997e7