Clean up handling of VR surface updates.

- Fix bug where adding a new hardware layer causes existing single-
  buffered surface layers to unlatch their buffer.
- Rename variables holding Composer pointers from hidl to composer
  to improve readability.
- Remove the arbitrary hardware layer limit, moving responsibility
  for efficient use of hardware layers to the VR display manager.

Bug: 64987282
Bug: 64756801
Test: Build system for multiple devices and manually test operation.
Change-Id: I8f05ca83830f62e15b18cd52df3014432c887681