Add Throwable.UNASSIGNED_STACK to well known fields

Replaces use of EmptyArray.STACK_TRACE_ELEMENT with
Throwable.UNSASSIGNED_STACK to match changes in Java code.

Bug: 35910877
Test: make checkbuild and run CtsLibcoreTestCases
Change-Id: Idc14ce8610ec12ce776d87befdbb9a3cbfd9dbee