[Android passwords settings] Add a progress bar

This CL adds a progress bar to the password export flow.

The progress bar is shown if the user already completed
reauthentication and export confirmation but the passwords
are still being serialized in the background. The user
can cancel the whole export with a Cancel button next
to the progress bar. The screenshot is at
https://crbug.com/788701#c22, the mocks at
go chrome-pwd-export-mocks-android (Google only).

This CL also merges the existing flags mExportOptionSuspended,
mExportRequested and mExportConfirmed into a single mExportState
to improve the clarity of the whole process.

Bug: 788701
Change-Id: I1857b135df255124a1507139095f36eca7c3dc3d
Reviewed-on: https://chromium-review.googlesource.com/881019
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#533335}
6 files changed