Ignore the app half's build outputs, which the last commit added

.gitignore covered the Rust half's target/ only, so `git add -A` swept in
about 190 files of Gradle intermediates -- caches, lint models, .class and
.dex files. Removed from tracking and ignored, in a new commit rather than
by rewriting what was already pushed.

.gitignore

@@ -1 +1,5 @@
 server/target/
+
+# Gradle build outputs for the app half.
+app/build/
+.gradle/