Bluetooth: refactor add `is_host_managed_ccc`

This check is repeated many times in `gatt.c`, with various comments.
This patch refactors these occurrences into a named function.

Resolves: https://github.com/zephyrproject-rtos/zephyr/issues/41789



Signed-off-by: Aleksander Wasaznik <aleksander.wasaznik@nordicsemi.no>