Enables detailed logging during variable speed playback.

This commit adds support for dynamically enabling logging in the
variable speed library if log.tag.VariableSpeed is set.

After this change, you can enable logging by running:

adb shell setprop log.tag.VariableSpeed 1

Logging will be disabled after reboot or after running:

adb shell setprop log.tag.VariableSpeed ""

The logging in the library is quite verbose, so we do not want to enable
it by default.

Bug: 5253004
Change-Id: I3addde5552093eb0c4926b67245d578a7da0fb7a