Reconcile WorkSource parcel and unparcel code.

Prior to this CL, WorkSources would Parcel their list of WorkChains as
-1 if null, or the size of the list followed by the list itself if
non-null. When reading it back in, on the other hand, they would check
if the size was positive, and only then read the list from the Parcel.
This works for all cases except when the WorkSource has an empty but
non-null list of WorkChains as the list would get written to the parcel,
but then never read on the other side.

If parceling a list was a no-op when empty this wouldn't be an issue,
but it must write at least its size into the parcel to know how many
elements to extract. In the empty list case, this single element is left
unread as the size is not positive which essentially corrupts any future
items read from that same parcelable.

Bug: 220302519
Test: atest android.security.cts.WorkSourceTest#testWorkChainParceling
Change-Id: I2fec40dfced420ca38e717059b0e95ee8ef9946a