NetworkChangeNotifierAutoDetect: cache current network state

This change factors out the connectivity manager API call from
NetworkChangeNotifierAutoDetect#getCurrentNetworkState() into a separate
method updateCurrentNetworkState(), which stores the result in
mNetworkState. getCurrentNetworkState() now just returns the current
value of mNetworkState.

All the callsites of getCurrentNetworkState() were updated to also call
updateCurrentNetworkState() beforehand, so the end result should be the
same as before.

This is a preparation change for https://crrev.com/c/5018956, where
unnecessary system calls will be eliminated by only calling
getCurrentNetworkState() where we know the state is sufficiently
up-to-date (currently planned to be limited to WebView startup).

Bug: 1493005, b:311417317
Change-Id: I4307dd5deb3a33b4a99253d88a588ee6c914fa53
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5106538
Commit-Queue: Oksana Zhuravlova <oksamyt@chromium.org>
Reviewed-by: Min Qin <qinmin@chromium.org>
Reviewed-by: Adam Rice <ricea@chromium.org>
Reviewed-by: Alexander Timin <altimin@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Cr-Commit-Position: refs/heads/main@{#1257390}