Avoid supported ABI list containing "unknown".

If one of the ABI list system properties is undefined/empty (as
ro.product.cpu.abilist64 is on a 32-bit only device), getString returns
"unknown", which ends up creating a 1-element array with "unknown" as a
member. Fix this to instead just get the empty string and split that
into a 0-element array.

Change-Id: I0d0a54eb06bb04427bcf0487e2a16d4180b81116