Transition SerialManager to @EnforcePermission

Use the new @EnforcePermission annotation for SerialService.

Test: Manually inspect ISerialManager.java, the permission checks are
      correctly generated.
Test: Run SerialChat app as regular app, SecurityException triggered
      when launching the app.
Test: Run SerialChat as priv-app. Access is granted, the app executes
      normally.
Bug: 197828948
Change-Id: Ide1d2809f2226f7cd84efa0d364dc78da726f294