nfc(app): Retain NfcCharging class in proguard config

This seems to be getting stripped out since it's behind a statically
disabled trunk stable flag in `-next` config.

Bug: 320173161
Bug: 320194986
Test: Verified NFC stack works now.
Change-Id: I7d951a2567749a79a7e9ae885bfc77fc1ed8bd61