BumbleBluetoothTests: Fix reconnectExistingClient flake

Fix GattClientTest#reconnectExistingClient flake where
the gatt callback was sometimes being called to notify
of disconnection at the end of the test. The end of the
test and the disconnection were racing. This is fixed
by explicitly disconnecting at the end of the test.

Bug: 322517596
Flag: TEST_ONLY
Test: atest BumbleBluetoothTests:android.bluetooth.GattClientTest
Change-Id: I428a3c6230aa139fc0479c1fcb5b09dd97686f57