Added PriorityDump helper util

Utility function to help services dump data in buckets (CRITICAL, HIGH,
NORMAL). Function parses --dump_priority argument to select which
sections to dump.

BUG: 27429130

Test: mmm -j32 frameworks/native/services/utils && \
      adb sync data && adb shell /data/nativetest/prioritydumper_test/prioritydumper_test && \
      adb shell  /data/nativetest64/prioritydumper_test/prioritydumper_test && \
      printf "\n\n#### ALL TESTS PASSED ####\n"

Change-Id: Ic529e136acfa4df82653b6b648cd540dada3ee96