Assert for a smooth width animation in auto enter

In gesture nav mode, auto entering PiP can initially
make the layer smaller before it gets larger.
However, asserting for area alone is not reliable at all,
since the area fluctuates due to crop and scale animations
going in the opposite directions.

For cases where source rect hint is null a more reliable assertion
is to check that the width of the PiP layer first decreases and then
increases.

Note that we still allow a margin of error of 1px, since around the time
of handoff between gesture nav task view simulator and
SwipePipToHomeAnimator, crop can get a bit smaller and scale can get a
bit larger if swiped aggressively - this can produce off-by-1 errors for
width too.

Allowing for this margin of error should still leave us capable of
catching most of the regressions for this enter PiP case. We do still
want to ideally come up with a better way of asserting animation
smoothness however (see b/363080056).

Bug: 293133362
Flag: EXEMPT bugfix
Test: atest AutoEnterPipOnGoToHomeTest
Test: atest AutoEnterPipWithSourceRectHintTest
Change-Id: I6588e81a6f20dcac444e4e7564917dc5889f913f