Support ahead-of-time compilation on Android in the engine (flutter/engine#2614)

This include build system changes for selecting Dart's precompiler mode
plus a way to locate and load the precompiled snapshot library from an
Android application

engine/src/flutter/sky/tools/gn

@@ -67,7 +67,6 @@ def to_gn_args(args):
           aot = False
         else:
           # The iOS simulator snapshot is host targetted
-          gn_args['dart_target_arch'] = ios_target_cpu
           aot = True
     elif args.target_os == 'fnl':
       gn_args['target_os'] = 'fnl'
@@ -91,6 +90,8 @@ def to_gn_args(args):
         gn_args['target_cpu'] = 'x64'
 
     if aot:
+        gn_args['flutter_aot'] = True
+        gn_args['dart_target_arch'] = gn_args['target_cpu']
         if args.debug:
             gn_args['dart_runtime_mode'] = 'profile'
         else:
@@ -98,6 +99,8 @@ def to_gn_args(args):
     else:
         gn_args['dart_runtime_mode'] = 'develop'
 
+    gn_args['flutter_product_mode'] = (gn_args['dart_runtime_mode'] == 'release')
+
     if args.target_sysroot:
       gn_args['target_sysroot'] = args.target_sysroot