Add recent disk write speed metrics to Disk Stats

Report disk write speed via dumpsys diskstats. Write speed provided by
storaged, which benchmarks current write speed every hour.

Fixes: 62393328
Test: manual (After at least an hour of use:
              adb shell dumpsys diskstats --proto > temp.pb
              printproto --proto2 --raw_protocol_buffer \
	      --message=android.service.diskstats.DiskStatsServiceDumpProto \
	      --multiline \
	      --proto=frameworks/base/core/proto/android/service/diskstats.proto \
	      ./temp.pb | grep benchmarked_write_speed_kbps)
Change-Id: Ic86406d6ce43d3962719af10f45288130c13b1a2