NuPlayer::GenericSource::onPrepareAsync():Change lock order

mLock and mDisconnect lock order interleaving,
which has potential risk of deadlock if other code locks/unlocks them in a different order.

Bug: 135211710
Test: (1) android.hardware.camera2.cts.FastBasicsTest#testCamera1
      (2) android.media.cts.MediaPlayerFlakyNetworkTest

Change-Id: I9486f2c6780c2f246786299a3d6c7c2f7295b768