Optimize calls to Trace for animators

Previously, ValueAnimator would always call into the Trace class to
log start/end events. When the animator is an ObjectAnimator, this
call necessitated building a new String to capture the animated property
name. This fix puts the calls to Trace inside a check for isTagEnabled(),
to ensure that we only bother building the trace name when tracing is
actually enabled.

Change-Id: I56ef093f3b67b31a19c861f9d1e44a84341edf53