base

Clean up and document app transition animation loading.

We had three different sources of information about where the
animated window will start/end: appFrame, containingFrame,
containingWidth/Height. We can merge these into one, because
containingWidth/Height can be calculated from containingFrame and
appFrame is actually used only once and the containingFrame is not
used in the same scenario.

Also, documentation about the frames and insets. The rest is fairly
simple, but frames and insets need explanation of how they are used.

Finally, simplify naming.

Bug: 25246833

Change-Id: I0accc2f23792bca4e1b9941007c6249745e023ce