Update Android SDK to 26 (#25856)

* Remove support for Android versions less than 24

Matter requires RIO, which is not supported on versions < 26.

* Raise Android API version from 21 to 26

* Use 0.6.51 vscode image for android builds

---------

Co-authored-by: Andrei Litvin <andy314@gmail.com>
diff --git a/gn_build.sh b/gn_build.sh
index 18473f3..9f6c765 100755
--- a/gn_build.sh
+++ b/gn_build.sh
@@ -126,7 +126,7 @@
     echo
     echo "Hint: Set \$ANDROID_HOME and \$ANDROID_NDK_HOME to enable building for Android"
     echo "      The required android sdk platform version is 21. It can be obtained from"
-    echo "      https://dl.google.com/android/repository/android-21_r02.zip"
+    echo "      https://dl.google.com/android/repository/platform-26_r02.zip"
 fi
 
 echo