Adds --dynamic and --interpreter flags to
tools/gn. These flags result in engines with
properties as follows:

--dynamic:
- JIT targeting native code on Android and
  DBC on iOS

--interpreter
- Target DBC even if running on Android.

For example:

gn --android --dynamic --interpreter --runtime-mode release

Will generate an engine:
- Without Dart asserts
- Without Observatory
- With JIT compililation to DBC

into out/android_dynamic_release_dbc