Make the dev icon distinct from the nightly one

Summary

Summarize the bug encountered concisely.

From !1410 (comment 3167950), we should revert the MR firefox-android!39 (merged) as it is causing issues with rebases and also makes it hard to tell apart debug and nightly builds.

Environment

Which operating system are you using? For example: Debian GNU/Linux 10.1, Windows 10, Ubuntu Xenial, FreeBSD 12.2, etc. Which installation method did you use? Distribution package (apt, pkg, homebrew), from source tarball, from Git, etc.

Relevant logs and/or screenshots

From !1410 (comment 3166391), build errors after rebasing

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> Task :app:processFenixNightlyResources FAILED

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:processFenixNightlyResources'.
> A failure occurred while executing com.android.build.gradle.internal.res.LinkApplicationAndroidResourcesTask$TaskAction
   > Android resource linking failed
[...]
     org.mozilla.fenix.app-main-189:/mipmap-anydpi-v26/ic_launcher_alternative.xml:7: error: resource drawable/ic_launcher_foreground (aka org.torproject.torbrowser_nightly:drawable/ic_launcher_foreground) not found.
     org.mozilla.fenix.app-main-189:/mipmap-anydpi-v26/ic_launcher_alternative.xml:8: error: resource drawable/ic_launcher_monochrome (aka org.torproject.torbrowser_nightly:drawable/ic_launcher_monochrome) not found.
     org.mozilla.fenix.app-main-189:/mipmap-anydpi-v26/ic_launcher_round_alternative.xml:7: error: resource drawable/ic_launcher_foreground (aka org.torproject.torbrowser_nightly:drawable/ic_launcher_foreground) not found.
     org.mozilla.fenix.app-main-189:/mipmap-anydpi-v26/ic_launcher_round_alternative.xml:8: error: resource drawable/ic_launcher_monochrome (aka org.torproject.torbrowser_nightly:drawable/ic_launcher_monochrome) not found.