Add support for prebuilt jars

third_party/guava/BUILD.gn

 import("//build/config/android/rules.gni")
 
 android_library("guava_javalib") {
   chromium_code = false
   deps = [
     "//third_party/jsr-305:jsr_305_javalib"
   ]
   java_files = [
     "src/guava/src/com/google/common/hash/BloomFilter.java",
     "src/guava/src/com/google/common/hash/Murmur3_32HashFunction.java",
@@ skipping 383 matching lines @@
     "src/guava/src/com/google/common/eventbus/Subscribe.java",
     "src/guava/src/com/google/common/eventbus/package-info.java",
     "src/guava/src/com/google/common/eventbus/AllowConcurrentEvents.java",
     "src/guava/src/com/google/common/eventbus/DeadEvent.java",
     "src/guava/src/com/google/common/eventbus/AnnotatedHandlerFinder.java",
     "src/guava/src/com/google/common/eventbus/HandlerFindingStrategy.java",
     "src/guava/src/com/google/common/eventbus/SynchronizedEventHandler.java",
     "src/guava/src/com/google/common/eventbus/EventBus.java",
     "src/guava/src/com/google/common/eventbus/AsyncEventBus.java",
   ]
+
+  proguard_preprocess = true
+  proguard_config = "proguard.flags"
 }