Fix missing GattService variable linkage

When scanManagerRefactor flag is enabled, `startGattProfileService()` is
never called and hence, the variable `mGattService` was never assigned.
Fixing that, fix the connecting issue as `getBluetoothGatt()` now no
longer returns null.

See go/scan-manager-refactor for more details.

Bug: 313335632
Bug: 267361243
Bug: 327503826
Bug: 343116685
Flag: com.android.bluetooth.flags.scan_manager_refactor
Test: m com.android.btservices
Change-Id: Ief870e097c643c60ff3249b1bf1e66e394285125