Cleanup DoH statistics when DoH is not supported in the private DNS mode.

Test steps:
a. Default enable strict mode.
b. Turn on WiFi and Mobile data.
c. Swtich private DNS mode from strict mode to automatic mode.
d. Expect to see DoH statistics only on WiFi.
e. Switch private DNS mode from automatic mode to off mode.
f. Expect to see no DoH statistics showing up on both WiFi and Mobile
data.

Tests will be done in a follow-up CL.

Bug: 201735918
Test: Check output in dumpsys dnsresolver
Change-Id: Ie4e67c3967778ada480cb2b5f9222cde5d2c705e