WifiMigration: Add helper to perform softap.conf to XML conversion

For softap configuration, we need to perform 2 levels of conversion:
a) AOSP softap.conf format to AOSP WifiConfigStoreSoftap.xml format
b) OEM specific customization to AOSP WifiConfigStoreSoftAp.xml format.

Moving a) from within the wifi stack to WifiMigration to allow OEM's to
adapt their customizations to the legacy conf files for b).

Bug: 149418926
Test: atest android.net.wifi
Test: Device boots up and able to turn on hotspot.
Test: Manual tests:
a) Flash Q build
b) Add hotspot config
c) Upgrade to RVC build with this CL.
d) Ensure all data has migrated over.
e) Modified hotspot config and reboot.
f) Ensured old data has not migrated over.

Change-Id: I6d900bd621873a0df81aaa17cf46ed153b073289