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When hotseat enters extended mode from home launcher mode, hotseat's
bounds undergo the following changes:
(1) The origin of the hotseat bounds moves to left.
(2) The hotseat bounds become wider.

Meanwhile, the hotseat background's origin is related to hotseat bounds.
Denote the background origin by |background_origin|; denote the hotseat
bounds by |bounds_rect|. Then  background_origin|.x is calculated
roughly by the equation below:
(1) |padding_inset| := (|bounds_rect|.width - space_taken_by_icons) / 2
(2) |background_origin|.x := padding_inset + |bounds_rect|.x

|bounds_rect|.x becomes smaller while |padding_inset| becomes bigger. It
is the reason why the hotseat bounds move back and forth.

In this CL, the custom layer element is created for the transition
between home launcher mode and extended mode. It ensures that the
hotseat background varies linearly in screen coordinates.

Bug: 1042911
Change-Id: Id91ce3ecf2bbee8509a3f794c6e943c6bf1d6ef3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2225590
Commit-Queue: Andrew Xu <andrewxu@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#776552}