Visualizer: use actual sampling rate of the instance

Visualizer.getSamplingRate method was always returning
the same value. This is not valid since the actual
sampling rate depends on the audio stream where
the Visualizer is instantiated.

Bug: 162727518
Test: atest CtsMediaAudioTestCases:VisualizerTest
Merged-In: Ia2fc453ce8b6b0e11e61bc00c764f822b39cf951
Change-Id: Ia2fc453ce8b6b0e11e61bc00c764f822b39cf951