Fix GATT client leakage when scan is throttled (1/2)

Currently, scan throttling happens after client is registered, but
before the scan is started. This might lead to scan client being leaked.
This patch fixed that by moving check before client registration.

Bug: 64887233
Test: manual
Change-Id: I22ae624a0c51110cb69679f796926e3b2b36d0ac
Merged-In: I22ae624a0c51110cb69679f796926e3b2b36d0ac
(cherry picked from commit ee02e1ce)