Add PreloadManager snippets, and update media3 to 1.8.0

[asolovay]

Submitting doc snippets used for the new PreloadManager documentation. Had to bump the Media3 version up to 1.8.0 for some of the new APIs in that doc.

Once we've submitted this in Github, I'll update my docs to pull the code snippets in from there.

[snippet-bot]

Here is the summary of changes.

You are about to add 7 region tags.

• misc/src/main/java/com/example/snippets/PreloadManagerKotlinSnippets.kt:34, tag android_defaultpreloadmanager_MyTargetPreloadStatusControl
• misc/src/main/java/com/example/snippets/PreloadManagerKotlinSnippets.kt:69, tag android_defaultpreloadmanager_createPLM
• misc/src/main/java/com/example/snippets/PreloadManagerKotlinSnippets.kt:78, tag android_defaultpreloadmanager_addMedia
• misc/src/main/java/com/example/snippets/PreloadManagerKotlinSnippets.kt:86, tag android_defaultpreloadmanager_invalidate
• misc/src/main/java/com/example/snippets/PreloadManagerKotlinSnippets.kt:98, tag android_defaultpreloadmanager_getAndPlayMedia
• misc/src/main/java/com/example/snippets/PreloadManagerKotlinSnippets.kt:117, tag android_defaultpreloadmanager_removeItem
• misc/src/main/java/com/example/snippets/PreloadManagerKotlinSnippets.kt:124, tag android_defaultpreloadmanager_releasePLM

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