Increase NNAPI VTS single-test timeout to 20m.

SaveToCache_TOCTOU/PrepareFromCache_TOCTOU loops 100 times to generate
and reload compilation cache. Some drivers may hit the 60-second
timeout.

Bug: 160197150
Test: Add a test sleeping for 2min to NNAPI 1.2/1.3 VTS
      and run with vts-tradefed
Change-Id: Id232f4954180d8d7a9b91f8d2e4e6f2ee4640ed7