Update AndroidPredictiveBackNavDecorator to handle its back animation

[stagg]

• Use None for the backwards AnimatedContent transition spec to avoid any collision with the predictive back
  • Reimplementing it on the content directly
• Added some animation for the elevation and corner radius of the "shared element" style predictive back
• Fully reset the state on back cancel

Screen_recording_20260331_150345.mp4

[ZacSweers]

@stagg anything else needed for this?

[stagg]

Not a fan of how the animations were completely ignoring the transition spec, so removed them. So the core race condition still exists. Think I might need to dig into a custom AnimatedContent for all this to cooperate correctly.