Use new unwinder for offline in libbacktrace.

libbbacktrace changes:
- Completely rewrite the BacktraceOffline class to use the new unwinder.
- Modify the test data to save ucontext_t data instead of unw_context data.
- Convert the previous tests from unw_context data to ucontext_t data.

Bug: 65682279

Test: New unit tests pass in libunwindstack.
Test: All offline tests continue to pass.
Change-Id: I540345c304b20199d46deeb0349a0638a0f3ab2f
diff --git a/libbacktrace/BacktraceCurrent.cpp b/libbacktrace/BacktraceCurrent.cpp
index d61b281..f6f4423 100644
--- a/libbacktrace/BacktraceCurrent.cpp
+++ b/libbacktrace/BacktraceCurrent.cpp
@@ -64,7 +64,7 @@
   return bytes;
 }
 
-bool BacktraceCurrent::Unwind(size_t num_ignore_frames, ucontext_t* ucontext) {
+bool BacktraceCurrent::Unwind(size_t num_ignore_frames, void* ucontext) {
   if (GetMap() == nullptr) {
     // Without a map object, we can't do anything.
     error_.error_code = BACKTRACE_UNWIND_ERROR_MAP_MISSING;