Add metrics for password export on Android, part 1

This CL adds the following metrics to the code for exporting passwords
from Android settings:
* PasswordManager.TimeReadingExportedPasswords
* PasswordManager.ExportPasswordToCSVResult

It also extends the enumeration for the latter to capture some
Android-specific errors.

More details about the metrics are in the design doc [1].

(Remaining TODOs for metrics, not addressed here:
Reporting PasswordManager.ExportedPasswordsPerUserInCSV and the user
actions.)

[1]
https://docs.google.com/document/d/1miKr2x0PTNIKgt3RQeur51ICQ66uszlYAmFXJONbG_0/edit?ts=5a313898#heading=h.chwovfmlf7pk

Bug: 788701
Change-Id: Ide5b1c908e7bd7a1240e1114487dab3636a5d315
Reviewed-on: https://chromium-review.googlesource.com/909108
Reviewed-by: Christos Froussios <cfroussios@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Theresa <twellington@chromium.org>
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#538059}
4 files changed