Add event metric for wifi health changes

This change adds a config for logging changes in wifi health
(cl/372232212).

Bug: 178132373
Test: Ran heavyweight-debug with this metric enabled to check that
      changes in wifi health are being collected and displayed.
Change-Id: I518897518b9f5427531b8c8d2f84112b148cff71
3 files changed
tree: d150a2ff197be24aa0c15f5f6647b05457ea7fc0
  1. build/
  2. docs/
  3. emu_test/
  4. libraries/
  5. scripts/
  6. tests/
  7. utils/
  8. PREUPLOAD.cfg