Paywalls: Fix proguard rules kotlinx serialization (#1356)

### Description
After adding kotlin serialization in the paywalls branch, looks like the
integration tests started failing to compile with the error:
```
R8: Missing class java.lang.ClassValue (referenced from: void kotlinx.serialization.internal.ClassValueCache$initClassValue$1.<init>(kotlinx.serialization.internal.ClassValueCache) and 3 other contexts)
```
In order to fix this issue, we are adding a `dontwarn` instruction to
the proguard rules of the library temporarily so it can compile.

After investigating why this is needed, I noticed
https://github.com/Kotlin/kotlinx.serialization/blob/ba38b7099e502c16efd30386d18174e3a7aefb3a/rules/common.pro#L32
and Kotlin/kotlinx.serialization#2119. But I
believe this shouldn't be needed from those comments... However, I
believe it should be safe to add it temporarily.

purchases/consumer-rules.pro

@@ -3,3 +3,5 @@
 -dontwarn com.google.errorprone.annotations.CanIgnoreReturnValue
 -dontwarn com.google.errorprone.annotations.Immutable
 -dontwarn com.amazon.**
+# Adding temporarily to fix issue after adding kotlin serialization
+-dontwarn java.lang.ClassValue