Implement puller for new SurfaceFlinger dmabuf stats.

Involves pushing most of the logic down out of StatsPullAtomService
and into KernelAllocationStats.cpp, as well as changing some method
signatures along the way.

At the end of this work, two new fields in atoms.proto will be
populated; these will allow us to tell how much of SurfaceFlinger's
dmabuf memory usage is due to buffers shared with other processes
and how much is due to private internal buffers.

Test: statsd_testdrive 10105
Test: adb shell dmabuf_dump
Bug: 220084049
Change-Id: Ia41cfd693d3db2bd06083801075cae290f67221f