VolumeShaper: Fixes for updated Cts test

1) Limit number of VolumeShapers that can be created.
   a) 16 system VolumeShapers
   b) 16 application/client VolumeShapers
2) Return proper volume before VolumeShaper is started.
3) Consistent xOffset definition used internally:
   a) this is now always the position on the volumeshaper curve
      which can go backwards if in REVERSE.
   b) normalized time is always forward going and is scaled
      to 0.f and 1.f depending on progress relative to
      the curve's duration.
4) Fix replace method.
5) Add comments.

Test: Use updated CTS VolumeShaperTest
Bug: 37536598
Change-Id: I837ab2a481adc0abbd3f1338bfe2cb79831b11fa