Fix a crash happening on a context menu.

When an activity gets a callback for onContextItemSelected, it first
gets dibs on handling it, and then it propagates the call to -every-
fragment it is tied to. This poses a problem since RootsFragment and
DirectoryFragment don't necessarily handle each other's menu items well,
which can cause NPEs. This is some temporary fix for it.

Bug: 30092676
Change-Id: I3f268c9c9580504b78581ad92a24c9a99c4a0daf