Leave from the current root when the root is removed.

This is needed to handle removable roots.

1. Add a listener to RootCache.
2. BaseActivity handles roots change and leaves from the current
   directory if the current root is removed.

BUG=26316940
Change-Id: I41921fe17782fa1a2d9e58f55ea84326b8f1d3c0
2 files changed