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/IWifiStaIface.hal b/wifi/1.0/IWifiStaIface.hal
index 96dc54a..43c3126 100644
--- a/wifi/1.0/IWifiStaIface.hal
+++ b/wifi/1.0/IWifiStaIface.hal
@@ -180,7 +180,6 @@
    * for the provided band. These channels may be specifed in the
    * |BackgroundScanBucketParameters.frequenciesInMhz| for a background scan
    * request.
-   * Must fail if |StaIfaceCapabilityMask.BACKGROUND_SCAN| is not set.
    *
    * @param band Band for which the frequency list is being generated.
    * @return status WifiStatus of the operation.
@@ -192,7 +191,7 @@
    *         |WifiStatusCode.ERROR_UNKNOWN|
    * @return frequencies vector of valid frequencies for the provided band.
    */
-  getValidFrequenciesForBackgroundScan(StaBackgroundScanBand band)
+  getValidFrequenciesForBand(WifiBand band)
       generates (WifiStatus status, vec<WifiChannelInMhz> frequencies);
 
   /**