Suppress Guava-related ProGuard/R8 warnings

I didn't copy-paste the whole of
https://github.com/google/guava/wiki/UsingProGuardWithGuava because
this line seems relevant based on our current usage.

Lots of that file seems to relate to classes that are strongly
discouraged on Android:
https://github.com/google/guava/wiki/Android#specifics

I've only added this to the `common` module, since everyone that uses
ExoPlayer must depend on that. This avoids duplicating this line into
every module that has a Guava dependency.

Also remove some other warning suppressions that are defined in both
`core` and `common`.

Issue: #7904
PiperOrigin-RevId: 332203086

RELEASENOTES.md

@@ -5,6 +5,9 @@
 *   Data sources:
     *   Add support for `android.resource` URI scheme in `RawResourceDataSource`
         ([#7866](https://github.com/google/ExoPlayer/issues/7866)).
+*   Core library:
+    *   Suppress Guava-related ProGuard/R8 warnings
+        ([#7904](https://github.com/google/ExoPlayer/issues/7904)).
 
 ### 2.12.0 (2020-09-11) ###
 

library/common/proguard-rules.txt

@@ -4,3 +4,6 @@
 -dontwarn org.checkerframework.**
 -dontwarn kotlin.annotations.jvm.**
 -dontwarn javax.annotation.**
+
+# From https://github.com/google/guava/wiki/UsingProGuardWithGuava
+-dontwarn java.lang.ClassValue

library/core/proguard-rules.txt

@@ -71,8 +71,3 @@
 -keepclasseswithmembers class com.google.android.exoplayer2.source.smoothstreaming.SsMediaSource$Factory {
   <init>(com.google.android.exoplayer2.upstream.DataSource$Factory);
 }
-
-# Don't warn about checkerframework and Kotlin annotations
--dontwarn org.checkerframework.**
--dontwarn kotlin.annotations.jvm.**
--dontwarn javax.annotation.**