[Fast Pair] Add Debug Metadata Flag

Adds a developer-only flag that allows debug-only Fast pair metadata to
show notifications. This would include test devices and demo devices.

to /etc/chrome_dev.conf and seeing debug device notifications.

Fixed: b/277094082
Test: Tested manually by adding "--enable-features:FastPairDebugMetadata"
Change-Id: I48ff6a22357403f4e58eb0ab77236d0a20d54550
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4409861
Commit-Queue: Daniel Classon <dclasson@google.com>
Reviewed-by: Jack Shira <jackshira@google.com>
Cr-Commit-Position: refs/heads/main@{#1127784}