[Android passwords settings] Add reauthentication for export

Mocks: go chrome-pwd-export-mocks-android (Google-internal only)

Screencast after the change: https://crbug.com/788701#c3

The CL adds a reauthentication step before the user can trigger
exporting passwords.

In addition to that, the CL also wraps the check for screen lock
availability in ReauthenticationManager.isScreenLockSetUp, to
make it possible to override for testing.

Also, this CL adds two tests for viewing passwords, because one
of earlier iterations of this CL broke something for the viewing
passwords feature and such tests would have caught it.

Bug: 788701
Change-Id: Ibfa37192e26bc5797236ba9378aa1eb0f17f79cc
Reviewed-on: https://chromium-review.googlesource.com/797453
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522061}
6 files changed