MAC Randomization: Store Randomized MAC Address in WifiConfiguration

Store randomized MAC address for each network in mMacAddress field, so
that we don't have to create a new MAC address when connecting to a
saved network.

Bug: 63905794
Bug: 71548300
Test: Unittest
Change-Id: I05a50d3c3bc94e5ac4a0ec7cbd1f192a6d4c0b11