SurfaceFlinger: fix flakiness in IdleTimerTest

Increase the timeout of waiting for reset callback to avoid flakinessas
a result of poor scheduling.

Test: adb shell /data/nativetest64/libsurfaceflinger_unittest/libsurfaceflinger_unittest --gtest_filter=IdleTimerTest* --gtest_repeat=1000
Change-Id: Ib87aebab4474baaa6583b9481137a268fafed368