Use this for offline stack data so that it's not necessary to
copy any data around.
Add unit tests for the new object.
Bug: 77258731
Bug: 74354410
Test: Ran libunwindstack/libbacktrace unit tests.
Change-Id: I9b0f25d9520c96e64aedef5f295423c60ddb3488
(cherry picked from commit 6633b0ca09)
