[lacros] Fix libmojo_core.so path in mojo/core/BUILD.gn

libmojo_core.so is generated by the a shared_library rule, which always
outputs the .so file inside:
${root_build_dir}/${toolchain_used_to_compile} directory.

Bug: 1129223
Change-Id: Ib2937510363ccc4db7df09d000d260d344aed4a3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2419149Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Yuke Liao <liaoyuke@chromium.org>
Cr-Commit-Position: refs/heads/master@{#808622}

mojo/core/BUILD.gn

@@ -217,16 +217,16 @@ if (is_chromeos || is_linux || is_android || is_win) {
     }
 
     copy("shared_library_arc32") {
-      sources = [ "${root_out_dir}/${android32_toolchain}/libmojo_core.so" ]
-      outputs = [ "${root_out_dir}/libmojo_core_arc32.so" ]
+      sources = [ "${root_build_dir}/${android32_toolchain}/libmojo_core.so" ]
+      outputs = [ "${root_build_dir}/libmojo_core_arc32.so" ]
       deps = [
         ":shared_library(//build/toolchain/android:${android32_toolchain})",
       ]
     }
 
     copy("shared_library_arc64") {
-      sources = [ "${root_out_dir}/${android64_toolchain}/libmojo_core.so" ]
-      outputs = [ "${root_out_dir}/libmojo_core_arc64.so" ]
+      sources = [ "${root_build_dir}/${android64_toolchain}/libmojo_core.so" ]
+      outputs = [ "${root_build_dir}/libmojo_core_arc64.so" ]
       deps = [
         ":shared_library(//build/toolchain/android:${android64_toolchain})",
       ]