OOBE: Fix hiding current_screen_ crashes in WizardController

In the WizardController, there are multiple places where
`current_screen_->Hide()` is called before setting `current_screen_`
to `nullptr`. For these instances `current_screen_` should be checked
to be pointing to a screen first before calling the `Hide()` method.

This is the second fix for this issue, to minimize the probability
of this issue re-occurring in the future, `ResetCurrentScreen()` method
is introuced.

Bug: b:290653733
Change-Id: I0c8eeb1c4c2afd215a2460bc189af88f496adce9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4969785
Commit-Queue: Osama Fathy <osamafathy@google.com>
Reviewed-by: Bohdan Tyshchenko <bohdanty@google.com>
Cr-Commit-Position: refs/heads/main@{#1214231}