Dismiss target v2

Current dismiss flow
- On bubble drag, show gradient with "X Dismiss"
- When bubbles get close to the gradient, show X-in-circle

This change
- Removes gradient and "X Dismiss"
- Shows X-in-circle (with 70% scrim inside) on bubble drag
- Reduces bubble_dismiss_encircle_size to spec (56dp => 52dp)
- Removes duplicate mDismissContainer in BubbleStackView

Fixes: 136199542
Test: manual (dismiss target has scrim, shows on bubble drag)
Test: manual (dismiss target behaves consistently for consecutive drags)
Test: atest SystemUITests
Change-Id: I21101ca9f2eb1b3c53b613231557f9f7936fddfa