Fix -Wunused-variable compiler warning

frameworks/base/tools/aapt2/link/ManifestFixer.cpp:341:23: error: unused variable 'attr' [-Werror,-Wunused-variable]
  341 |   if (xml::Attribute* attr = el->FindAttribute(xml::kSchemaAndroid, "glEsVersion")) {
      |                       ^~~~

Test: presubmit
Change-Id: Icec4737a5f8076173a2aff54532a79fa123df326