Bug 1831287 - Update Private Mode color scheme

According to the figma color scheme, the color given to layer 3
was wrong, therefore it has been changed to the correct one
from the document.

Figma color scheme: https://shorturl.at/mAS48

app/src/main/java/org/mozilla/fenix/theme/FirefoxTheme.kt

@@ -222,7 +222,7 @@ private val lightColorPalette = FirefoxColors(
 private val privateColorPalette = darkColorPalette.copy(
     layer1 = PhotonColors.Ink50,
     layer2 = PhotonColors.Ink50,
-    layer3 = PhotonColors.Ink50,
+    layer3 = PhotonColors.Ink90,
 )
 
 /**

app/src/main/res/values/colors.xml

@@ -136,7 +136,7 @@
     <!-- Card background, Menu background, Dialog, Banner -->
     <color name="fx_mobile_private_layer_color_2">@color/photonInk50</color>
     <!-- Search -->
-    <color name="fx_mobile_private_layer_color_3">@color/photonInk50</color>
+    <color name="fx_mobile_private_layer_color_3">@color/photonInk90</color>
     <!-- Homescreen background, Toolbar -->
     <color name="fx_mobile_private_layer_color_4_start">@color/photonPurple70</color>
     <!-- Homescreen background, Toolbar -->