Move the test page contents for WebView PaymentRequest to its own file

The AwPaymentRequestServiceTest.java file has become large and
monolithic, which makes changing and reviewing it more difficult.

This patch moves the contents of the test web page being used in
AwPaymentRequestServiceTest.java to its own file
PaymentRequestTestWebPageContents.java  in the "test_support_java" build
target.

After this patch, the AwPaymentRequestServiceTest.java is simpler and
other test files can reuse the new PaymentRequestTestWebPageContents
class, instead of creating their own.

This patch is a move or extraction of the test page contents. This patch
does not change the interface for PaymentRequestTestWebPageContents
significantly. Improvement to this interface are being tracked in
https://crbug.com/401515769.

Fixed: 401515287
Bug: 401515769
Change-Id: I91314a0724d21868d76c4a4b881d1bd8f46ce37d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6335119
Auto-Submit: Rouslan Solomakhin <rouslan@chromium.org>
Reviewed-by: Stephen McGruer <smcgruer@chromium.org>
Commit-Queue: Rouslan Solomakhin <rouslan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1430927}