[CrOS Cellular] Fix Cellular Setup and SIM Unlock issues.

Previously, if the user enters the Mobile data settings subpage
via QS to add a mobile network, then subsequently visits a subpage
of the Mobile data settings subpage, and traverses back to the mobile
data settings subpage, the add a mobile network dialog reappears because
the showCellularSetup param is still present in the URL.

The same behavior occurs for the showSimLockDialog param when the user
enters the Sim unlock settings subpage via QS.

Manually tested for the case of cellular setup, but still waiting on
a physical SIM card to test SIM lock.

Fixed: 1199476, 1199950
Change-Id: Ie9c05190c1601291c0b492a5ecbc0672edd66180
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2865766
Reviewed-by: Kyle Horimoto <khorimoto@chromium.org>
Commit-Queue: Regan Hsu <hsuregan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#878475}