Use String#lines() rather than String#split()

Use the more predictable behaviour of String#lines() to split the stdout
string as an example of how it should be parsed. String#split() has
slightly surprising behaviour if stdout is empty as it results in an
array containing an empty string.

Test: atest RemoteProvisioningShellCommandTest
Bug: 281584054
Change-Id: I06796772cc58007fefb5d2089ae709965c6576b1