Add task content animation property

Add API to animate task item content transition from one set of task
content (icon, thumbnail) to another. To do this, we provide two things
to the caller: startContentAnimation which allows the caller to set the
icon, thumbnail, and label to animate to and the
CONTENT_TRANSITION_PROGRESS property which the caller can use to control
the transition progress.

We will eventually hook this up to onBindViewHolder for the task adapter
when there is a data change event to prepare to animate content in.
Currently it still changes immediately.

Bug: 114136250
Test: Builds
Change-Id: I16e9b757ee91be54fe8cba6780b399e3cc313e3e