Stamp fuchsia packages with api level (flutter#29998)

BUG: https://bugs.fuchsia.dev/p/fuchsia/issues/detail?id=87813

As part of the fuchsia platform versioning we are stamping all of
our packages with a target api level which can be read by our assembly
tools. We would like to be able to update the target api level with a
roller so I added a file which can is read by the gn tool. I am open
to suggestions about how to change this is we do not want to add
file read calls to our gn script.

tools/fuchsia/fuchsia_archive.gni

@@ -151,6 +151,8 @@ template("_fuchsia_archive") {
       rebase_path(manifest_json_file, root_build_dir),
     ]
 
+    assert(fuchsia_target_api_level != -1,
+           "Must set a target api level when creating an archive")
     if (fuchsia_target_api_level != -1) {
       args += [
         "--api-level",

tools/fuchsia/target_api_level

@@ -0,0 +1 @@
+7

tools/gn

@@ -406,6 +406,10 @@ def to_gn_args(args):
 
     if args.fuchsia_target_api_level is not None:
       gn_args['fuchsia_target_api_level'] = args.fuchsia_target_api_level
+    elif args.target_os == 'fuchsia':
+      # Read the default target api level from a file so we can update it with a roller
+      with open(os.path.join(os.path.dirname(__file__), 'fuchsia/target_api_level')) as f:
+        gn_args['fuchsia_target_api_level'] = int(f.read().strip())
 
     return gn_args