build-tools/26.0.0/apksigner does not exist, which completely breaks android_binary.
it appears that this was a bug in the build-tools 26.0.0 release, and should be fixed in the next release.
The android public tracking issue is https://issuetracker.google.com/issues/62696222
A workaround until a fixed Android SDK Build Tools 26.x build is available is to copy build-tools/25.0.3/apksigner* and build-tools/25.0.3/lib/apksigner.jar to build-tools/26.0.0/.
Android SDK Build Tools 26.0.1 has been published. It contains apksigner.
Due to another breaking change, a future Bazel release will drop support for build-tools versions prior to 26.0.1. See #3469 for context.
Most helpful comment
A workaround until a fixed Android SDK Build Tools 26.x build is available is to copy build-tools/25.0.3/apksigner* and build-tools/25.0.3/lib/apksigner.jar to build-tools/26.0.0/.