[Android settings] Disable password export if there are no saved passwords

If the settings do not show any saved passwords, the export button in
the menu should still be present, but disabled.

Note that the existence of "blacklisted" entries, i.e., entries where
the user asked Chrome to never save a password, does not influence
the state of the menu item, because those entries are not exported.

Mocks: go chrome-pwd-export-mocks-android (currently slide 10,
Google-internal only)

General design:

Bug: 788701
Change-Id: I1f5ad12eef9ef6ec1c9b0529c5b1ef16941c5298
Reviewed-on: https://chromium-review.googlesource.com/811186
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Cr-Commit-Position: refs/heads/master@{#522400}
2 files changed