Add a profile build mode flag for building APKs with AOT compilation (#3526)

This is currently hidden for development use.  It will select artifacts from
the android_{Debug,Release}_Deploy output in a local engine build.
diff --git a/packages/flutter_tools/lib/src/build_configuration.dart b/packages/flutter_tools/lib/src/build_configuration.dart
index 924ec31..abe98aa 100644
--- a/packages/flutter_tools/lib/src/build_configuration.dart
+++ b/packages/flutter_tools/lib/src/build_configuration.dart
@@ -15,11 +15,10 @@
   debug,
 }
 
-/// The type of build - `debug` or `release`.
-///
-/// TODO(devoncarew): Add a `profile` mode.
+/// The type of build - `debug`, `profile`, or `release`.
 enum BuildMode {
   debug,
+  profile,
   release
 }