Installer: remove dependency on JSON extension

* Remove one of the two uses of Xml::encodeJsVar(): to encode an
  unused JS variable dbTypes. The remaining use is only after
  installation is complete.
* Add an environmental check to ensure the JSON functions are
  available before attempting installation.

Bug: 54774
Change-Id: Ifb8859026b853ef3f328b796945a973c4add7eeb
Merge "Installer: remove dependency on JSON extension"