Added preview assets for search widget within all variants.
Verified as fixed in the latest Nightly build 67.0a1 and a developer builds with Google Pixel 3 XL.
Please note that the previews were as expected.
Checked the search_widget_preview assets which are to be included in the different build variants - https://searchfox.org/mozilla-central/source/mobile/android/branding
The assets used are the expected ones, using different logos depending on the build variant"
Nightly build 67.0a1 and a developer