gcstress: increase libcore timeout.

Diagnosing buildbot issues.  I am not sure if it is needed.

Bug: 138868214
Bug: 142039427
Test: run vogar for libcore.libcore.io
Change-Id: I201d4d0b4a85ed6edd685139da01f80da9fe29a3
diff --git a/tools/run-libcore-tests.sh b/tools/run-libcore-tests.sh
index 4c39445..2506c9b 100755
--- a/tools/run-libcore-tests.sh
+++ b/tools/run-libcore-tests.sh
@@ -198,6 +198,8 @@
 # the default timeout.
 if $gcstress && $debug && $device_mode; then
   vogar_args="$vogar_args --timeout 1440"
+elif $gcstress && $device_mode; then
+  vogar_args="$vogar_args --timeout 900"
 else
   vogar_args="$vogar_args --timeout 480"
 fi