Hardcode auto-provisioning in Gradle to off.

gradle.properties

@@ -1,6 +1,12 @@
 org.gradle.jvmargs=-Xmx12g -Xms256m -XX:MaxMetaspaceSize=1g
 android.useAndroidX=true
 android.experimental.androidTest.numManagedDeviceShards=4
+# We never want to use auto-provisioning, as it breaks reproducible builds.
+# This should not be a problem, because we never configure a "javaRepositories"
+# item to tell Gradle where to auto-provision the toolchain from, but adding
+# this flag adds an extra layer of protection.
+# See: https://docs.gradle.org/current/userguide/toolchains.html#sub:disable_auto_provision
+org.gradle.java.installations.auto-download=false
 
 # Uncomment these to build libsignal from source.
 # libsignalClientPath=../libsignal