Silence duplicate DNS prefetch requests

- IwlanDataService can issue upto 4 DNS prefetch requests when
WiFi connection comes online, as part of onCapabilitiesChanged()
and onLinkPropertiesChanged().
- No-op the duplicate DNS prefetch requests when a prefetch is already
running. Change is local and private to EpdgSelector class.

Bug: 229015509
Test: Live test on TMO. EpdgSelectorTests pass.
Change-Id: Ic3b016ab5cf72c612a723690cac7d289f43f402f
1 file changed