wifi: Duplicate getValidChannelsForBand in AP iface

This functionality is needed even for AP iface. So, duplicate this
method.

Changes in the CL:
1. Add getValidChannelsForBand() in IWifiApIface.
2. Rename StaBackgroundScanBan to WifiBand to make it a more generic
name.
3. Change the existing method name in IWifiStaIface to match with the
one in IWifiApIface.

While there,
Fix indentation in hidl_callback_util.h

Bug: 35663149
Test: Compiles
Change-Id: Iaeb6c323e13eedf2f98de92fea77e327c76ffa5b
diff --git a/wifi/1.0/default/wifi_legacy_hal.h b/wifi/1.0/default/wifi_legacy_hal.h
index ed020b0..c8fd5bd 100644
--- a/wifi/1.0/default/wifi_legacy_hal.h
+++ b/wifi/1.0/default/wifi_legacy_hal.h
@@ -175,7 +175,7 @@
       const on_gscan_results_callback& on_results_callback,
       const on_gscan_full_result_callback& on_full_result_callback);
   wifi_error stopGscan(wifi_request_id id);
-  std::pair<wifi_error, std::vector<uint32_t>> getValidFrequenciesForGscan(
+  std::pair<wifi_error, std::vector<uint32_t>> getValidFrequenciesForBand(
       wifi_band band);
   // Link layer stats functions.
   wifi_error enableLinkLayerStats(bool debug);