BluetoothGatt: Fix duplicate GATT services being added

When BluetoothGatt#discoverServices() is called right before the binder
method onSearchComplete() is called, then the BluetoothGatt instance
gets two duplicate GATT services.

This CL fixes this issue by changing the timing of clearing cache into
onSearchComplete().

Bug: 391997799
Bug: 391773937
Test: atest GattClientTest
Flag: com.android.bluetooth.flags. fix_bluetooth_gatt_getting_duplicate_services
Change-Id: I55ad412e0dbbeef8eeb7884a9ff04850f0ca8664