dumpstate: print DMA-BUF sysfs stats in bugreport

Dump DMA-BUF per-buffer/per-exporter/per-device stats in bugreport.

These statistics will be helpful to analyze memory-related issues like Low
Memory Kills and ANR events.

Measured over 5 cycles, the addition of the statistics appear to
approximately increase the size of bugreport.zip by 26KB.

Bug: 167709539
Test: adb shell am bug-report
Change-Id: I2aceef79c4ec97ad400cb250c0075004fe50cdc6