[Refactoring] Migrate resource_dirs => sources

This CL converts resource_dirs to sources for android_resources in
BUILD.gn files.

Details:
https://crbug.com/1026378
https://groups.google.com/a/chromium.org/forum/#!msg/chromium-dev/KAFsfaoq7y4/OC4v5sb9BQAJ

Bug: 1062263
Change-Id: I5726e7858e2dab3164b08a81151ab6a5d6fd6f15
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2137479Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Cr-Commit-Position: refs/heads/master@{#756670}

chrome/browser/android/examples/partner_browser_customizations_provider/BUILD.gn

@@ -5,7 +5,10 @@
 import("//build/config/android/rules.gni")
 
 android_resources("partner_browser_customizations_example_apk_resources") {
-  resource_dirs = [ "res" ]
+  sources = [
+    "res/values/bookmarks_icons.xml",
+    "res/values/strings.xml",
+  ]
   android_manifest = "AndroidManifest.xml"
   custom_package = "org.chromium.example.partnercustomizations"
 }