WifiScanner: Expose ScanData.getBandsScanned publicly

i) Rename from getBandScanned to getBandsScanned.
ii) Created a getBandsScannedInternal for usage within wifi stack (which
should not be guarded with SDK level check like the public API)

Bug: 172412541
Test: atest com.android.server.wifi.scanner
Change-Id: I4e3f9457da2f7cac5585a7f5343d535ef4083f4b
14 files changed