Fix flaky WifiTrackerTest

The rssiChangeBroadcastShouldUpdateConnectedAp uses two CountDownLatch instances to
synchronize the events on the main thread and the handler thread. The "latch" informs
the main thread that mockWifiManager.getConnectionInfo() has been called and allows it
to proceed, and "mAccessPointsChangedLatch" notes that onAccessPointsChanged is called.
But mAccessPointsChangedLatch was assigned after "latch" released the main thread,
so that on occasion the main thread could try to use it before this assignment, causing
the timeout.

Move the assignment of mAccessPointsChangedLatch to before the body of the test
to prevent this race.

Bug: 70163367
Test: runtest --path frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/WifiTrackerTest.java
Change-Id: I62c1ef9a2155ffbd08b5e07e94723b28102d1f6d