Update build-tools and platfrom to API 28.

This is the last version before AndroidX needed to be used.  Piet still doesn't support the full Android-X suite yet.

PiperOrigin-RevId: 375549962
Change-Id: Id9da0d9ac2d447304130964333ceb60e65084e05

WORKSPACE

diff --git a/WORKSPACE b/WORKSPACE
index fdd696b..b072adc 100644
--- a/WORKSPACE
+++ b/WORKSPACE
@@ -1,5 +1,6 @@
 android_sdk_repository(
-    name = "androidsdk"
+    name = "androidsdk",
+    api_level = 28,
 )
 
 load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")