[fuchsia] Update Fuchsia API version to 19

Bug: b/347292659
Change-Id: I16422488566c91b2efc1c06192583eb3937dd4b4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/372185
Auto-Submit: Jonny Wang <jonnywang@google.com>
Commit-Queue: Derek Xu <derekx@google.com>
Reviewed-by: Derek Xu <derekx@google.com>

tools/gn.py

@@ -235,7 +235,7 @@ def ToGnArgs(args, mode, arch, target_os, sanitizer, verify_sdk_hash,
             gn_args['arm_float_abi'] = floatabi
             gn_args['arm_use_neon'] = True
     if gn_args['target_os'] == 'fuchsia':
-        gn_args['fuchsia_target_api_level'] = 18
+        gn_args['fuchsia_target_api_level'] = 19
 
     gn_args['is_debug'] = mode == 'debug'
     gn_args['is_release'] = mode == 'release'