Prevents the broadcastReceiver unregistered when it's not registerd

WindowMagnificationManager monitors the screen state.
And when requestConnection(false) is called before
requestConnection(true), the broadcastReceiver would be unregistered
even it is not registered yet. And it may lead to crash.

Bug: 167804868
Test: atest WindowMagnificationManagerTest
Change-Id: Ib61b00afe79d21a5e944affe8c5492aef90f0399