Provider-level changes for implementing direct eject of a root in Files app.

Several changes at different levels:
1. Introduction of ejectRoot(String) for DocumentsProvider
2. Introduction of ejectRoot(ContentResolver, Uri, String) for
DocumentsContract
4. Additional permission for MOUNT_UNMOUNT for ExternalStorageProvider
5. Implementation of ejectRoot(String) for External StorageProvider

Bug: 29584653
Change-Id: I28557af63259548784cf24d5b051eb06ad5193ca
(cherry picked from commit 2ccc18357d6741dde56edc4d5a2608f15f4b9078)