Pass Feed library's proguard config through its build target's proguard_config parameter

R=skym@chromium.org
CC=agrieve@chromium.org

Change-Id: I565a3cd2e8af0c90e5727ac9c61eec631d6ec264
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1773890Reviewed-by: Sky Malice <skym@chromium.org>
Commit-Queue: Pavel Yatsuk <pavely@chromium.org>
Cr-Commit-Position: refs/heads/master@{#691299}

third_party/feed/BUILD.gn

@@ -105,6 +105,9 @@ android_library("feed_lib_java") {
     "//third_party/android_deps:javax_inject_javax_inject_java",
     "//third_party/jsr-305:jsr_305_javalib",
   ]
+  if (!is_java_debug) {
+    proguard_configs = [ "src/src/main/java/com/google/android/libraries/feed/buildtools/proguard/r8_release.flags" ]
+  }
 }
 
 android_library("feed_conformance_test_lib_android_java") {