Android: Use experimental_use_r8 instead

Broken by revert of CL that introduced use_r8:
https://crrev.com/c/1394528

TBR=smaier@chromium.org

Bug: 918796
Change-Id: I1c059625de3ebc0482a3be79302c535035c17f12
Reviewed-on: https://chromium-review.googlesource.com/c/1394648Reviewed-by: Peter Wen <wnwen@chromium.org>
Reviewed-by: Sam Maier <smaier@chromium.org>
Reviewed-by: Egor Pasko <pasko@chromium.org>
Commit-Queue: Peter Wen <wnwen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#619646}

build/config/android/internal_rules.gni

@@ -3369,7 +3369,7 @@ if (enable_java_templates) {
 
         # Skip desugaring for debug builds using r8 to increase incremental
         # build speed. Release builds still need desugar.jar for binary size.
-        _using_r8 = !defined(invoker.proguard_jar_path) || use_r8
+        _using_r8 = !defined(invoker.proguard_jar_path) || experimental_use_r8
         _desugar = !(_using_r8 && is_java_debug) && _supports_android
         process_java_prebuilt(_process_prebuilt_target_name) {
           forward_variables_from(invoker,