Make staging manager lists private to GrGpu.

Use const accessors for iterating.
Move platform-indepenent work to GrGpu::disconnect().

Change-Id: I0acabb5b5fd27ddb47f4051f956ce8130174a8d0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/282267
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-by: Greg Daniel <egdaniel@google.com>
5 files changed