Suppress ProGuard warnings related to Guava's compile-only deps

Without these lines, ProGuard fails on the demo app (R8 works).

Also include some more `-dontwarn` lines from
https://github.com/google/guava/wiki/UsingProGuardWithGuava

Issue: #8103
PiperOrigin-RevId: 339050634

RELEASENOTES.md

@@ -6,6 +6,8 @@
     *   Suppress exceptions from registering/unregistering the stream volume
         receiver ([#8087](https://github.com/google/ExoPlayer/issues/8087)),
         ([#8106](https://github.com/google/ExoPlayer/issues/8106)).
+    *   Suppress ProGuard warnings caused by Guava's compile-only dependencies
+        ([#8103](https://github.com/google/ExoPlayer/issues/8103)).
 *   IMA extension:
     *   Upgrade IMA SDK dependency to 3.21.0, and release the `AdsLoader`
         ([#7344](https://github.com/google/ExoPlayer/issues/7344)).

library/common/proguard-rules.txt

@@ -7,3 +7,12 @@
 
 # From https://github.com/google/guava/wiki/UsingProGuardWithGuava
 -dontwarn java.lang.ClassValue
+-dontwarn java.lang.SafeVarargs
+-dontwarn javax.lang.model.element.Modifier
+-dontwarn sun.misc.Unsafe
+
+# Don't warn about Guava's compile-only dependencies.
+# These lines are needed for ProGuard but not R8.
+-dontwarn com.google.errorprone.annotations.**
+-dontwarn com.google.j2objc.annotations.**
+-dontwarn org.codehaus.mojo.animal_sniffer.IgnoreJRERequirement