SoundEffectsHelper: Update resource info after unloading

SoundEffectsHelper.mResources info needs to be reset
after unloading of samples. Lack of this cleanup is
benign when subsequent resource loading succeeds, but
in case it doesn't having stale info in mResources can
lead to confusion.

Test: atest android.media.cts.AudioManagerTest#testSoundEffects
Change-Id: Ia7a1cc4315bfcf05fa4af915c4ad690183ed4368