fwupd: Use named D-Bus types in message parsing

Replace GetDataSignature with GetDataType. The former returns a string,
while the latter returns an enum value. No change to functionality, just
a little easier to read.

BUG=none
TEST=chromeos_unittests --gtest_filter=FwupdClientTest.*

Change-Id: I3adcfcc7606a3ac29e6f5b8439836d129a09237b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5582293


Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Nicholas Bishop <nicholasbishop@google.com>
Reviewed-by: Jimmy Gong <jimmyxgong@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1313661}