[fuchsia] use the api-level from gn-sdk (flutter#54740)

https://crrev.com/c/5805927 provides the default api-level in gn-sdk, so
as long as we have the latest gn-sdk / skia setup, the api-level would
be updated automatically.

Bug: [347373793](http://b/347373793)

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Author: zijiehe-google-com

build/config/fuchsia/gn_configs.gni

@@ -21,12 +21,6 @@ declare_args() {
 # TODO(zijiehe): Remove the override and move it into declare_args once the one
 # in //build/config/sdk.gni being removed. - b/40935282
 
-# The target API level for this repository. Embedders should override this
-# value to specify the API level the packages produced from this repository
-# should be targeting, e.g. in their top-level //.gn file. A value of -1
-# means that no API level will be passed to the tools that consumes it.
-fuchsia_target_api_level = 22
-
 # The SDK manifest file. This is useful to include as a dependency
 # for some targets in order to cause a rebuild when the version of the
 # SDK is changed.

tools/fuchsia/build_fuchsia_artifacts.py

@@ -178,12 +178,13 @@ def CopyToBucket(src, dst, product=False):
 
 
 def ReadTargetAPILevel():
-  filename = os.path.join(os.path.dirname(__file__), '../../build/config/fuchsia/gn_configs.gni')
+  filename = os.path.join(os.path.dirname(__file__), 'gn-sdk/src/gn_configs.gni')
   with open(filename) as f:
     for line in f:
+      line = line.strip()
       if line.startswith('fuchsia_target_api_level'):
         return line.split('=')[-1].strip()
-  assert False, 'No fuchsia_target_api_level found in //flutter/build/config/fuchsia/gn_configs.gni'
+  assert False, 'No fuchsia_target_api_level found in ' + filename
 
 
 def CopyVulkanDepsToBucket(src, dst, arch):

tools/fuchsia/build_fuchsia_artifacts_test.py

@@ -8,10 +8,7 @@
 class BuildFuchsiaArtifactsTest(unittest.TestCase):
 
   def test_read_fuchsia_target_api_level(self):
-    # It's expected to update this test each time the fuchsia_target_api_level
-    # in //flutter/build/config/fuchsia/gn_configs.gni is changed, so we don't
-    # accidentally publish the artifacts with an incorrect api level suffix.
-    self.assertEqual(build_fuchsia_artifacts.ReadTargetAPILevel(), '22')
+    self.assertGreater(int(build_fuchsia_artifacts.ReadTargetAPILevel()), 21)
 
 
 if __name__ == '__main__':