Wifi: Add constant and utility method for bands

This commit adds a constant WIFI_BAND_ALL to refer to bit mask
for all bands.
It also adds a method to check on if previous scan covers all
supported bands.

Bug: 156053158
Test: atest android.net.wifi
Change-Id: I426dd3940b7efd67eaf6e12f862a1e3f0ebc177a