[FedCM] Refactor title updates to correctly update in multi IDP

Currently, the title is not updated correctly for multi IDP. We fix this
by computing whether multiple IDPs are being shown in updateSheet() and
by setting the idp for display and icon in showAccountsInternal()
instead of showAccounts() since the former is the one which is invoked
when the back button is pressed.

Demo: https://issues.chromium.org/issues/392142580#comment16

Bug: 392142580
Change-Id: I3b07e023007e85f78b8b32a3a0b5eece8fe1b4c4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6352821
Commit-Queue: Nicolás Peña <npm@chromium.org>
Reviewed-by: Zachary Tan <tanzachary@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1432452}