[Snackbar] Refactor mandatory system gesture insets handling

[pubiqq]

• Fixes Snackbar flickering that occurs when the anchor view is updated off-screen and gestureInsetBottomIgnored = false.
• Fixes Snackbar vertical offset when target parent has bottom padding.
• Removes a double layout pass request when calling the margin update with gestureInsetBottomIgnored = false and anchorViewLayoutListenerEnabled = false.
• Removes endless layout pass requests when calling the margin update with gestureInsetBottomIgnored = false and anchorViewLayoutListenerEnabled = true.

Fixes #3446, fixes #4326.

[drchen]

I think we post to update gesture insets for a reason. @dsn5ft do you have an idea why we did that in the first place?

[dsn5ft]

I don't really remember, maybe updateMargins() was originally not called as often as it is now? @pubiqq do you know why the post() is not necessary anymore and you can call calculateExtraBottomMargin() directly in updateMargins()?

[pubiqq]

I'm pretty sure .post() was added just because it was the easiest way to add mandatory system gesture inset support. There are no objective reasons to keep this approach.