There’s a bug in Android Studio that doesn’t allow fallback to older SDK sources when new sources are not available. This causes Android Studio to decompile the sources and make for painful debugging. A workaround suggested by Jake Wharton here makes a copy of Android 25 and renames it to Android 26. Execute the following commands to resolve the issue temporarily:

cd $ANDROID_HOME/sources
cp -R android-25 android-26
cd android-26
vi/emacs/nano package.xml
# Find/replace 25 with 26
vi/emacs/nano source.properties
# Find/replace 25 with 26