defaults: Set default systemd job stop timeout to 15 seconds

This avoids the phone getting stuck for a long time on shutdown,
trying to stop a misbehaving service. If a service doesn't stop
gracefully after 15 seconds, there's little chance it ever will.
5 jobs for stop-timeout in 4 minutes and 33 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #367373
librem5
shellcheck

00:00:23

 
  Package
passed #367374
librem5
package-pureos-byzantium

00:00:53

passed #367375
librem5:arm64
package-pureos-byzantium:arm64

00:02:03

 
  Test Package
passed #367376
librem5
autopkgtest-pureos-byzantium-package

00:03:16

passed #367377
librem5
lintian-pureos-byzantium-package

00:01:14