ChannelEditorDialog: use @dimen/large_dialog_width

ChannelEditorDialog (which uses notif_half_shelf as the root layout) was
displaying with layout_width wrap_content, which meant the dialog was
snugly wrapped around the app icon, name, and global notification
toggle.

We already have a convenient resource, @dimen/large_dialog_width, for
dialogs like this, so use it!

Bug: 309133675
Flag: NA
Test: manual
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:f47606f122049bd73ce39b1e449cbda794a74395)
Merged-In: Ic3871b721b9ecf37abc6d92b0378003806f20ff2
Change-Id: Ic3871b721b9ecf37abc6d92b0378003806f20ff2