[BNPL][Clank] Use BnplStrategy in BnplManager

Refactors BnplManager to use BnplStrategy for determining the next steps
in platform-dependent parts of the BNPL flow. Also adds corresponding
method definitions and implementations into BnplStrategy variants.

Design: go/autofill-clank-bnpl

Bug: 430575808
Change-Id: I5bf1c9a891958ad78318994ae5fb84a746676b5f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6832634
Reviewed-by: Vinny Persky <vinnypersky@google.com>
Reviewed-by: Viplav Kadam <viplavkadam@google.com>
Commit-Queue: Wilson Low <wilsonlow@google.com>
Cr-Commit-Position: refs/heads/main@{#1499848}