[Fast Pair] Don't call CancelPairing for successful pair attempts

Adds logic to the destructor of FastPairPairerImpl to ensure we don't
call CancelPairing for successful pair attempts, which can lead to
immediate disconnects and other issues.

Change-Id: I8eb279a39114abac4381450c08feed744d837436
Fixed: b/256023856
Tested: Tested manually on DUT.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3991155
Reviewed-by: Juliet Lévesque <julietlevesque@google.com>
Commit-Queue: Daniel Classon <dclasson@google.com>
Cr-Commit-Position: refs/heads/main@{#1071258}