Change the assets path adaptive to device/deviceless tests.

The specification of assets path would not affect the test run, but
would take effect when some screenshots mismatch and we'd like to upload
the screenshots. Since device and deviceless tests do not share
screenshot folder, we would have to adaptively specify that.

Test: atest -c SpaRoboRNGTests
Bug: 300278210
Change-Id: I09f10834d16eabcc10a0f027d525c8259d3d58e9