Add private gpu and total dmabuf size to SystemMemory atom

Changes:
- Transition to measuring dmabuf export size (ion
allocations are most recently implemented via dmabufs anyway, the total
ion size will be a subset of the total dmabuf exported memory)

- For supported kernels (5.4+) that have sysfs dmabuf accounting,
account for private GPU driver memory vs GPU driver dmabuf memory.

- Adjust the accounted memory calculations based on the above.

Bug: 149462065
Bug: 172201884
Test: build, statsd_testdrive
Change-Id: Ie4e4d0b4fc4495896374fbebc70e29e32469b5e3