Fix a crash in onAvailable() network callbacks

ConnectivityManagerDelegate.getNetworkCapabilities() is non-reentrant,
because it may crash and retry. So in MyNetworkCallback.onAvailable(),
the `capabilities` variation may be inited as nullptr, but passes
MyNetworkCallback.ignoreConnectedNetwork() check.
MyNetworkCallback.ignoreConnectedInaccessibleVpn() doesn't need to
invoke ConnectivityManagerDelegate.getNetworkCapabilities() again.

Bug: 350362785
Change-Id: Ia22cacab3669e320510dfc06c06af11d495b85f3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5670233
Reviewed-by: Oksana Zhuravlova <oksamyt@chromium.org>
Auto-Submit: 余佳 <yujia.1019@bytedance.com>
Commit-Queue: Kenichi Ishibashi <bashi@chromium.org>
Reviewed-by: Kenichi Ishibashi <bashi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1326510}