Adding PreloadManager doc snippets

Adding snippets to support new PreloadManager docs. That requires bumping the Media3
version up to 1.8.0.

Author: asolovay

gradle/libs.versions.toml

@@ -60,7 +60,7 @@ maps-compose = "6.6.0"
 material = "1.14.0-alpha02"
 material3-adaptive = "1.1.0"
 material3-adaptive-navigation-suite = "1.3.2"
-media3 = "1.7.1"
+media3 = "1.8.0"
 # @keep
 minSdk = "35"
 okHttp = "4.12.0"