Add ScanController mock to LeAudioServiceTest

This is needed as the `mGattService` is mocked for `mAdapterService` but not `mScanController` when the flag `scan_manager_refactor` is on.

Bug: 378582911
Change-Id: I25a244d519563d3de2267318aa9568da7c9562e4
Test: atest GoogleBluetoothInstrumentationTests
Flag: Exempt; Unit test fix