stagefright: don't auto-drop temporal layers for unknown SVC streams

- SVC optimization requires knowin the number of layers which is
  using custom signaling. Don't drop temporal layers without that information.

Also SVC optimization fixes:

- Play back all layers immediately after an IDR frame (previous logic
  required leveling up one level at a time that only works for hierarchical
  B frames)
- Track current max layer-ID during dropping so we can react to upward
  changes in the allowed layers more quickly.
- Get up to 90% of display rate before dropping layers (up from 50%)

Bug: 27596987
Change-Id: Ia6c8363fc154c822a5b2a268e2f40678212a487c