Wifi: add APIs to set/get max supported link speed

Add the following APIs:
getMaxSupportedTxLinkSpeed()
setMaxSupportedTxLinkSpeed()
getMaxSupportedRxLinkSpeed()
setMaxSupportedRxLinkSpeed()

Bug: 147390882
Test: atest android.net.wifi
Test: manual test with various APs
Change-Id: I7f3268b2842783f99c0a43d8b4996d0a7dd46c6e