wifi: Add gscan API wrappers in WifiLegacyHal

The legacy gscan API's were designed to be used in the following
sequence:
a) Start the scan using |wifi_start_gscan|.
b) Scan Events are indicated to the caller via the provided
|on_scan_event| callback.
c) When one of the result events are received, the caller is expected
to retrieve the cached results via |wifi_get_cached_gscan_results|.
There are some extra knobs here to determine if the results need to be
flushed after fetch or not.
d) Any scan failures are also notified via the provided
|on_scan_event| callback.
e) Full scan results are delivered one by one via the provided
|on_full_scan_result| callback.

In our use case step (b) above is always followed by step (c), so these
2 steps have been merged together in the HIDL interface:
a) Start the scan using |IWifiStaIface.startBackgroundScan|.
b) Scan results are now directly delivered via
|IWifiStaIfaceEventCallback.onBackgroundScanResults| callback.
c) Any scan failures will be delivered via
|IWifiStaIfaceEventCallback.onBackgroundScanFailure| callback.
d) Full scan results are delivered one by one via
|IWifiStaIfaceEventCallback.onBackgroundFullScanResult| callback.

Bug: 31991459
Test: Compiles
Change-Id: I0870eae095a667eec1d8de75fe1cc04a1b5a0bd3