This removes direct file access from within flutter_tools
in favor of using `package:file` via a `FileSystem` that's
accessed via the `ApplicationContext`.
This lays the groundwork for us to be able to easily swap
out the underlying file system when running Flutter tools,
which will be used to provide a record/replay file system,
analogous to what we have for process invocations.
This change adds a top-level getBuildDirectory func and funcs for
android, aot, asset, ios build products.
Developers may now add a "build-dir" mapping to their
~/.flutter_settings (JSON format) config file. Output directory is
relative to the main flutter application directory.
This change also changes the default build directory for iOS builds to a
subdirectory of the configured build directory, 'build/ios' by default.
