Server 3.0: emmc-wait: Increase systemd timeout to match script

Commit adef9d0f3aca increased the timeout in `wait_emmc_available.sh`
to 900 seconds (15 minutes). However, the systemd unit `emmc-wait.service`
was left at its previous `TimeoutStartSec=330s`, causing systemd to
prematurely terminate the service before the script could finish waiting.

This change bumps `TimeoutStartSec` to 930 seconds (15.5 minutes),
synchronizing the service with the script and providing a 30-second
buffer so the script can gracefully timeout and log errors instead of
being abruptly SIGTERM-ed by systemd.

fusion2: n/a

Tested: Bitbake builds
Google-Bug-Id: 549235198
Change-Id: I6efca51cf48e8c7de098bee6feadbdba66f45d71
Signed-off-by: Vikram Gara <vikramgara@google.com>