TrackMetrics: Fix volume computation

Average volume should be weighted based on active time
not total lifetime.

Improve the numerical accuracy of average volume using
West's algorithm.

Remove unneeded variables.

Test: youtube, play/pause, adb shell dumpsys media.metrics
Bug: 233409482
Change-Id: I9fadb7287bd0bccbf5c74aad5a07eab277303d02