Add default proguard flags from guava

Guava 27.1 causes some warnings in R8, silence them.

Bug: 130306229
Test: m checkbuild
Change-Id: Ia9948d597538a16c5b7ab26a2e41eebde52e7427

core/proguard_basic_keeps.flags

@@ -66,5 +66,12 @@
 # See bug/20658265.
 # -dontwarn android.support.**
 
+# From https://github.com/google/guava/wiki/UsingProGuardWithGuava
+# Striped64, LittleEndianByteArray, UnsignedBytes, AbstractFuture
+-dontwarn sun.misc.Unsafe
+# Futures.getChecked (which often won't work with Proguard anyway) uses this. It
+# has a fallback, but again, don't use Futures.getChecked on Android regardless.
+-dontwarn java.lang.ClassValue
+
 # Less spammy.
 -dontnote