[Mullhet] p_m_util::GetMatchType supports grouped credentials.

This change modifies GetMatchType utility function to support grouped
credentials. The logic is modified in several places where this
function is used. The end goal of the CL is to make sure that grouped
credentials are handled correctly in all places where GetMatchType is
used.

This CL corresponds to the section called "Adding support for grouped
credentials" in go/chrome-mullhet-suggestion-generation.

Bug: b/331409076
Change-Id: Id02aa52a4adf0e7a486cf45223426ead3ed552af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5402732


Reviewed-by: Vasilii Sukhanov <vasilii@chromium.org>
Commit-Queue: Timofey Chudakov <tchudakov@google.com>
Cr-Commit-Position: refs/heads/main@{#1281139}