The material action bar is elevated by default. The following code sets the elevation to 0 and removes the shadow.
Depends on Phone and JavaObject libraries.
B4X:
Sub RemoveActionBarShadow
Dim p As Phone
If p.SdkVersion >= 21 Then
Dim jo As JavaObject
jo.InitializeContext
Dim elevation As Float = 0
jo.RunMethodJO("getActionBar", Null).RunMethod("setElevation", Array(elevation))
End If
End Sub
Depends on Phone and JavaObject libraries.