Open the destination dir when a copy notification is tapped.

- Cleanup: rename the "open copy destination" action to "pick copy
  destination", which better reflects what it does.
- Move DocumentsIntent from BaseActivity to Shared.
- Rename ACTION_BROWSE_DOCUMENT_ROOT to ACTION_BROWSE for general browsing.
- Use the new ACTION_BROWSE to open the copy destination when copy
  notifications are tapped.

BUG=23137963

Change-Id: I10480b45a16ce716febac5453cb5015d26bb0062