Reduce minimum time threshold on dumpstate_smoke_test

Sometimes bugreports finish slightly before the 30 second
threshold, causing test failures. This change reduces this
threshold to 20 seconds and fixes some misleading failure
logs.

Test: atest dumpstate_smoke_test
Bug: 211004508
Bug: 211488888
Change-Id: I37e7707df3237f39d099484a26f195a85f16767a