WifiConfigStore: Encrypt credentials for networks (2/4)

Plumb the EncryptionUtil & version info to the XmlUtil classes that
actually handle the serialization/deserialization of data.

Also, created a helper class to serialize/deserialize EncryptedData
class.

Bug: 140485110
Test: atest com.android.server.wifi
Change-Id: I92846f1fb63f3b85892750b195a535bcfdc03e2c
Merged-In: I92846f1fb63f3b85892750b195a535bcfdc03e2c
5 files changed