Add a MemoryOfflineBuffer object.

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 6633b0ca090dd3da45b8936d587a1316401e46ba)
6 files changed
tree: 613818ffe74f64a3b017ae7c811451e67fce83a0
  1. libbacktrace/
  2. libunwindstack/