Added extra validation for invalid paths.

In normal circumstances, StorageVolume can only be obtained through
StorageManager and hence it will contain a valid path internally. But
the path could be invalid in at least 2 occations:

- App maliciously changed it using reflection.
- Mounted volume was ejected.

This change improves OpenExternalDirectoryActivity so it returns a
RESULT_CANCELED upon receiveving an invalid path.

BUG: 27962875

Change-Id: Ide43968babaa37961c7b704bd289f44eac952e94