GattServiceTest: Avoid directly assigning member variables as mocks

GattServiceTest overwrites the member variables in the GattService,
which results their cleanup() not be called.
It resulted NPE in Broadcast receiver used in ScanManager.
This CL fixes this issue.

Bug: 291390459
Test: atest GoogleBluetoothInstrumentationTests
Ignore-AOSP-First: Automerge conflict btw. AOSP and this branch
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:a260fc77dae276fdf51d05fbb0ea0f0336c83bca)
Merged-In: I63d6f30b67a4e8b7f2f5702a5ea0687a84142115
Change-Id: I63d6f30b67a4e8b7f2f5702a5ea0687a84142115