Document current Android limitations for JDBC integration

The problem was originally reported in #1014, where using DataFrame JDBC/SQL APIs on Android leads to a runtime crash.

A couple of workarounds were discussed there, but full Android support for JDBC integration is planned only after the 1.0 release.

Before 1.0, we should clearly document the current status for Android users:

- JDBC integration is not fully supported on Android at the moment
- users may hit platform-specific runtime errors
- available workarounds should be described
- future work is planned after 1.0

This should be added to the FAQ and/or Android-related documentation so users understand the current limitations before trying to use JDBC integration on Android.