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

@@ -79,8 +79,8 @@ wearComposeMaterial3 = "1.5.0-beta04"
 wearOngoing = "1.0.0"
 wearToolingPreview = "1.0.0"
 webkit = "1.14.0"
-media3CommonKtx = "1.7.1"
-media3Exoplayer = "1.7.1"
+media3CommonKtx = "1.8.0"
+media3Exoplayer = "1.8.0"
 
 [libraries]
 accompanist-adaptive = "com.google.accompanist:accompanist-adaptive:0.37.3"