Parse system properties that are lists correctly.

String.split() returns a one-element array of the empty string if the
input is the empty string, not a zero-element array. Introduce a
getStringList function to handle this consistently in the cases where
system properties are parsed as comma separated lists.

The main result is that SUPPORTED_64_BIT_ABIS will now correctly be
empty on a 32-bit only device.

Change-Id: I6f74b48d0a6ced3cd6d49c05aad6a4b7e0b074d2