SystemServer: test race condition on enable

This race condition is no longer possible due to the threading
management of enable happening on the correct thread.
Adding test to prevent further regression

Test: atest ServiceBluetoothTests:BluetoothManagerServiceTest
Bug: 328846833
Flag: TEST_ONLY
Change-Id: I0275a64fb01ce3f685b8349370a4b5dbef585b0d