SoundEffectsHelper: Fix NPE when calling into SoundPool

The instance of SoundPool might get released as a result
of unloading sound effects. Required checks for the validity
of the SoundPool pointer were missing resulting in an NPE.

Bug: 159870337
Test: atest android.media.cts.AudioManagerTest#testSoundEffects
Change-Id: Ief4f7f0027a1f03f71d3db5af623df9ad3c37a1b