MB-8461: Change swap usage tracking

As the current swap file size can grow beyond the specified
base capacity, use committed bytes minus physical memory to
calculate current size of page file. Derive actual use using
fraction obtained by the swap percentage use counter. Removed
the now unused SystemUpTime query.

Note that this will not match "Page File" shown by task manager
because that is tracking max potential usage and not the actual
current usage, which is what we are reporting.

Change-Id: Ibf1b772822e46938570c6fd31b1c6c51a59fa48e