runtime: move gcPercent and heapMinimum into gcControllerState

These variables are core to the pacer, and will be need to be non-global
for testing later.

Partially generated via

rf '
    ex . {
	gcPercent -> gcController.gcPercent
	heapMinimum -> gcController.heapMinimum
    }
'

The only exception to this generation is usage of these variables
in gcControllerState methods.

For #44167.

Change-Id: I8b620b3061114f3a3c4b65006f715fd977b180a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/306600


Trust: Michael Knyszek <mknyszek@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Michael Pratt <mpratt@google.com>