Camera2 CTS: Fix too-low timeout in AllocationTest#testBlackWhite

A fixed 5-second timeout isn't long enough when dealing with devices
that support long exposure.

Ideally the timeout would be calculated based on the requested
exposure time, but for simplicity, simply increase it to 20 seconds.

Test: CTS test now passes on a device supporting ~4 second exposures
Bug: 38212789
Change-Id: Ib5f0840e341490bca2472fa243b5649224975d4a
1 file changed