* Enable flutter engine to also work with .*so files on android
We would like to be able to use native tools (e.g. simpleperf, gdb) with
precompiled flutter apps. The native tools work much better with *.so
files instead of the custom formats the Dart VM uses by default.
This CL adds support for being able to load the flutter app from an *.so
file on Android.
* Add sanity check to ensure we have either shared library or instruction snapshot (but not both)