* Add support for NDK discovery and add --prefer-shared-library option

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.

The reason for using blobs / instruction snapshots is that we do not
want to force flutter users to install the Android NDK.

This CL adds a `--prefer-shared-library` flag to e.g. `flutter build
apk` which will use the NDK compiler (if available) to turn the
precompiled app assembly file to an `*.so` file.  If the NDK compiler is
not available it will default to the default behavior.

* Rebase, add test for NDK detection, augment flutter.gradle with @Input for flag

* Use InMemoryFileSystem for test

* Remove unused import

* Address some analyzer warnings