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
diff --git a/scanning/scanner_impl.cpp b/scanning/scanner_impl.cpp
index 2c1b1d0..a4db6da 100644
--- a/scanning/scanner_impl.cpp
+++ b/scanning/scanner_impl.cpp
@@ -82,6 +82,7 @@
             << (int)interface_index_;
   scan_utils_->UnsubscribeScanResultNotification(interface_index_);
   scan_utils_->UnsubscribeSchedScanResultNotification(interface_index_);
+  valid_ = false;
 }
 
 bool ScannerImpl::CheckIsValid() {