Add metrics for password export on Android, part 2

This CL adds PasswordManager.ExportedPasswordsPerUserInCSV to the code
for exporting passwords from Android settings.

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

(Remaining TODO for metrics, not addressed here: user actions.)

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

Bug: 788701
Change-Id: I844f7506590d505b1d2e4671e21f5d363d73a2e7
Reviewed-on: https://chromium-review.googlesource.com/926527
Reviewed-by: Bernhard Bauer <bauerb@chromium.org>
Commit-Queue: Vaclav Brozek <vabr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#539946}
12 files changed