feat(android): elevation shadow support (#7136)

tns-core-modules/ui/button/button.android.ts

@@ -3,11 +3,16 @@
     paddingLeftProperty, paddingTopProperty, paddingRightProperty, paddingBottomProperty,
     Length, zIndexProperty, textAlignmentProperty, TextAlignment
 } from "./button-common";
+import { androidElevationProperty, androidDynamicElevationOffsetProperty } from "../styling/style-properties";
 import { profile } from "../../profiling";
 import { TouchGestureEventData, GestureTypes, TouchAction } from "../gestures";
+import { device } from "../../platform";
+import lazy from "../../utils/lazy";
 
 export * from "./button-common";
 
+const sdkVersion = lazy(() => parseInt(device.sdkVersion));
+
 interface ClickListener {
     new(owner: Button): android.view.View.OnClickListener;
 }
@@ -146,9 +151,28 @@ export class Button extends ButtonBase {
         org.nativescript.widgets.ViewHelper.setZIndex(this.nativeViewProtected, value);
     }
 
+    [androidElevationProperty.getDefault](): number {
+        if (sdkVersion() < 21) {
+            return 0;
+        }
+
+        // NOTE: Button widget has StateListAnimator that defines the elevation value and
+        // at the time of the getDefault() query the animator is not applied yet so we
+        // return the hardcoded @dimen/button_elevation_material value 2dp here instead
+        return 2;
+    }
+
+    [androidDynamicElevationOffsetProperty.getDefault](): number {
+        if (sdkVersion() < 21) {
+            return 0;
+        }
+
+        return 4; // 4dp @dimen/button_pressed_z_material
+    }
+
     [textAlignmentProperty.setNative](value: TextAlignment) {
         // Button initial value is center.
         const newValue = value === "initial" ? "center" : value;
         super[textAlignmentProperty.setNative](newValue);
     }
-}
+}