LeAudioService: Fix starting scanner

Make sure to retry when registration does not happen, and in other case
clear the callback which indicates scan is ongoing.

Bug: 283228408
Test: manual
Tag: #feature
(cherry picked from https://android-review.googlesource.com/q/commit:8c02fa7af535428884f57d88e68e479087bec83d)
Merged-In: I2399058997b20ea3534951b25c4a8169211d7052
Change-Id: I2399058997b20ea3534951b25c4a8169211d7052