Ensure smart suggestion background color is updated just after inflation

Whenever we inflate new smart suggestions we create a new SmartReplyView
instance with a default background color. We then use that default color
to color our smart suggestion buttons.

With this CL we instead fetch the background color of the owning
notification (just after the smart suggestions are inflated) to ensure
the colors of the smart suggestions are consistent with the color of the
notification.

Smart Suggestions color update flow:

Before this CL:
1. notification update
2. smart suggestions inflated - using
SmartReplyView.mDefaultBackgroundColor (white)
3. add SmartReplyView to NotificationContentView -> calls into
NotificationContentView.setBackgroundTintColor() now update smart
suggestions colors

After this CL:
1. notification update
2. smart suggestions inflated - using
SmartReplyView.mDefaultBackgroundColor (white)
3. [this CL] update smart suggestion colors
4. add SmartReplyView to NotificationContentView -> calls into
NotificationContentView.setBackgroundTintColor() now update smart
suggestions colors

Bug: 132838369
Test: manual, as per instructions in the bug
Change-Id: Ie6fe13b9d01faf4e19b71a15532a1b664617ecbb