cellular: pseudomodem: Make Modem.Reset recycle modem object.

Instead of asynchronously disabling and enabling during reset, the
pseudomodem object will now disappear from the DBus connection and
reappear with a new DBus path, followed by an Enable if necessary. The
Reset call returns immediately while the logic takes place
asynchronously, which is much closer to what the real modem manager
does.

BUG=chromium:238409
TEST=1. network_LTEActivate.
     2. Remote trybots.

Change-Id: I3b5df85272a02781028758b5d045204587744c4b
Reviewed-on: https://gerrit.chromium.org/gerrit/50247
Commit-Queue: Arman Uguray <armansito@chromium.org>
Reviewed-by: Arman Uguray <armansito@chromium.org>
Tested-by: Arman Uguray <armansito@chromium.org>
3 files changed