The entry goes into the .csproj, not AndroidManifest.xml.
This can also be set in the IDE.
Click Project > Project Name Options.
In the left-hand tree, select Build > Mono for Android Build.
In the right-hand pane, select the Advanced tab.
In the Internationalization section, click the checkbox next to west.
Note: Make sure you're changing options for your Release build, not the Debug build.
Within Visual Studio:
Click Project > Project Name Properties.
In the Application tab, look in the Application properties > Additional supported encodings section (it's on the right).
Click the checkbox next to West.