Set icon size to 48x48 in the monochrome version.

[roundhill]

This PR is a small tweak for the monochrome icon to be a bit smaller to 48x48 so that it has some nice padding around the outside.

This is recommended in the guidelines: https://developer.android.com/develop/ui/views/launch/icon_design_adaptive#design-adaptive-icons

Before:

After: (matches up with Jetpack nicely too!)

To test

• Enable color palettes and toggle Apply palette to app icons.
• The Simplenote icon should look nice!

[wpmobilebot]

You can test the changes on this Pull Request by downloading an installable build (simplenote-android-installable-build-pr1634-15f09de-018d8054-6746-437c-8934-ca4634f4ce63.apk), or scanning this QR code:

[roundhill]

Thank you @AmandaRiu ;)