Use ThreadLocal for static Animator variables.

Certain fields in Animator are statics, like the list of current animations and the
main handler. However, since there may be >1 UI thread per process, these should really
be ThreadLocal variables, so that they are local to each UI thread. For example,
most animators will cause an invalidation in the view hierarchy, which can only
happen in the UI thread for that view.

Change-Id: I42be61c781ffac97b527f78ce1ffc2e0cdc42999