Increase NNAPI VTS single-test timeout to 20m.

Meet timeout when run NNAPI VTS on some A35 CPU device.
Enlarge the single-test timeout from 1m to 20m.

Bug: 175510656

Test cases:
VtsHalNeuralnetworksV1_0TargetTest
VtsHalNeuralnetworksV1_1TargetTest

Change-Id: I869208c79ff4c1d1928603a5630f456915928957