Disable kotlin -checkdiscard rule

kotlin-stdlib is included transitively as a library jar by way of
androidx.annotations, which means we cannot ensure associated classes
are discarded by R8. Disable the related `-checkdiscard` proguard rule
until that inclusion is resolved, avoiding spurious warnings in build
output.

Bug: 302383328
Test: m
Flag: NA
Change-Id: I7514e6841735e95e1a16d42d6a6e134103ecb108

core/proguard/kotlin.flags

@@ -10,7 +10,9 @@
 
 # Kotlin DebugMetadata has no value in release builds, these two rules, will
 # allow AppReduce to strip out DebutMetadata.
--checkdiscard interface kotlin.coroutines.jvm.internal.DebugMetadata
+# TODO(b/302383328): Restore the below checkdiscard after resolving transitive
+# inclusion of kotlin-stdlib from androidx.annotation library deps.
+# -checkdiscard interface kotlin.coroutines.jvm.internal.DebugMetadata
 -assumenosideeffects class kotlin.coroutines.jvm.internal.DebugMetadataKt {
   *** getDebugMetadataAnnotation(...);
 }