Add pick ActionHandler test.

Just basic at this point, only covers initial root loaded when picking copy dest.
Pulled OnRootsChangedTask out of BaseActivity.

Address followups from: https://googleplex-android-review.git.corp.google.com/#/c/1505194/

Change-Id: I2bbf418c8c9c6ad34e6f751e4a69e0f4243cc3c9