Copy libcore.utils.HexEncoding into Wifi module

@CorePlatformApi's cannot be depended on by Wifi
Mainline module. Thus create a copy and migrate
usages over.

Bug: 145409537
Test: atest FrameworksWifiApiTests
Change-Id: If4133bdd9ac9ca005538a03592462d2e1365c0d8
7 files changed