Provide an accessibility tree snapshot

BUG=497258

Provide an accessibility tree snapshot when requested. This CL
contains a frameworks class that is taken from preview release
and will be removed when the public SDK is available.

Depends on https://codereview.chromium.org/1246163003/

Review URL: https://codereview.chromium.org/1246823003

Cr-Original-Commit-Position: refs/heads/master@{#339968}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 650a6fa6cabd2c5a825af031e5f471cc4c7db60c

secondary/third_party/android_tools/BUILD.gn

@@ -20,6 +20,10 @@ source_set("cpu_features") {
   configs += [ "//build/config/compiler:no_chromium_code" ]
 }
 
+android_library("preview_java_sources") {
+  DEPRECATED_java_in_dir = "$android_sdk_root/sources/preview_sources/src"
+}
+
 android_java_prebuilt("android_gcm_java") {
   jar_path = "$android_sdk_root/extras/google/gcm/gcm-client/dist/gcm.jar"
 }