Migrate Leanback sample app to Jetpack Compose for TV

- Rename and modernize the main sample app to 'Leanback-Compose'.
- Upgrade the build system to Gradle 8.7, AGP 8.6.0, and JDK 21, targeting SDK 35.
- Fix UI issues including focus traps in Authentication/Search and initial focus in Vertical Grid/Error screens in the Compose version.
- Modernize the legacy 'Leanback' app (migrated to Media3, targeting SDK 35) to keep it compile-able and runnable on Android 14+.
- Disable background recommendation updates on launch in the legacy app to prevent startup crashes on modern SDKs.
- Update the README.md with a comprehensive migration guide and new 1080p side-by-side comparison screenshots.

Author: Paul Lammertsma

AccessibilityDemo/build.gradle

@@ -3,7 +3,7 @@
 buildscript {
     repositories {
         google()
-        jcenter()
+        mavenCentral()
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:4.1.3'
@@ -15,7 +15,7 @@ buildscript {
 allprojects {
     repositories {
         google()
-        jcenter()
+        mavenCentral()
     }
 }
 

ClassicsKotlin/build.gradle

@@ -21,7 +21,7 @@ buildscript {
 
     repositories {
         google()
-        jcenter()
+        mavenCentral()
     }
 
     dependencies {
@@ -37,7 +37,7 @@ buildscript {
 allprojects {
     repositories {
         google()
-        jcenter()
+        mavenCentral()
     }
 }