WifiDiagnostics: require verbose mode for ringbuffers

Only enable ringbuffers if the user has enabled verbose
mode via developer options. This is due to the CPU load
required to push the ringbuffers up to the framework,
with the current ringbuffer HAL.

Along the way:
1) Update some test comments to reflect new behavior
2) Remove unnecessary local variable in existing test

Bug: 68283533
Test: runtests.sh -e class com.android.server.wifi.WifiDiagnosticsTest (on walleye)
Manual Test:
- (enable verbose mode)
$ adb shell reboot
$ adb shell bugreport >/tmp/foo.bug 2>/dev/null; </tmp/foo.bug grep --text -A1 ring-buffer | egrep -v '^ring-buffer|^--$|^$' | wc -l
  4
Manual Test:
- (disable verbose mode)
$ adb shell reboot
$ adb shell bugreport >/tmp/foo.bug 2>/dev/null; </tmp/foo.bug grep --text -A1 ring-buffer | egrep -v '^ring-buffer|^--$|^$' | wc -l
  0
Change-Id: I3d68a80cb298ffee1b00cbc9b8c7756d08a7a943
2 files changed