Increase timeout for VtsHalGraphicsComposerV2_4TargetTest

The current timout of 5 mins is not enough for
setActiveConfigWithConstraints to complete on devices
which have ~30 display modes and which can't do seamless
switching. The test needs 8 mins to complete locally --
increasing the timeout to 15 to have some margin.

Bug: 188686850
Test: atest VtsHalGraphicsComposerV2_4TargetTest
Change-Id: I07cf1825c23e2467852d5cd93436eb9dee23b408