[CrOS Cellular] Add NetworkHandler init check to EuiccStatusUploader.

If the ESimPolicy flag is enabled and there is no NetworkHandler check,
many unit_tests tests fail.

Bug: b/207589472
Change-Id: I2f27f963c5c3b3ccd2077792c8c1ecac9e313f7b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3414071


Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Reviewed-by: Jason Zhang <jiajunz@google.com>
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#963087}