[Payment Handler][Android] Change payment method event.

This patch adds ability for the payment handler to fire a
"paymentmethodchange" event in the corresponding PaymentRequest object.
This patch is for Android only. This feature is behind a flag.

Spec:
https://w3c.github.io/payment-handler/#changepaymentmethod-method

Intent to implement:
https://groups.google.com/a/chromium.org/d/msg/blink-dev/-ObfNnnJjLo/pPsUD8CUCAAJ

Chrome status entry:
https://chromestatus.com/feature/5698314223747072

Bug: 884680
Change-Id: I044469f6353e49796e33468bc9345ca607524f3a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1592220
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Danyao Wang <danyao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#660126}
24 files changed