[Mfill Android] Add manual generation button in the bottom sheet

This change is part of the Android Password Manager UI.

This CL adds a button to the accessory sheet, meant to allow users
to manually trigger password generation. The change is flag guarded
by #manual-password-generation-android.

Bug: 835234
Change-Id: Ie651d51c53ee28a943fd06f6f93396a495720354
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1479996
Commit-Queue: Ioana Pandele <ioanap@chromium.org>
Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Reviewed-by: Friedrich [CET] <fhorschig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#658695}
19 files changed