Avoid double event listener registration in LatencyTester

Inside the start() method we were calling registerForBroadcasts()
which doesn't guard against duplicate registration.

Bug: 280052123
Test: Add logging to KeyguardUpdateMonitor.onFingerprintAuthenticated,
then run 'adb shell am broadcast -a
com.android.systemui.latency.ACTION_FINGERPRINT_WAKE' and examine
logcat to see that it was running twice without this fix

Change-Id: Ic7968454519ae2bf987eb690a6bacf402ea8afa6