wificond: Mark scanner impl instance invalid

The call to ScannerImpl.invalidate() does not reset the |valid_| field
currently.  This leads to unexpected behavior when any calls are made
on that instance after invoking invalidate().

Bug: 110378816
Bug: 121327323
Test: Repeatedly toggled wifi state and ensured we connected back.
Change-Id: I8da3e1d9284ec9e27f8a0980a669611369810c34
(cherry picked from commit 7b332f6c5f43d455e948fab6b705b9985b061348)