Small behavioral changes to the unwinder.

- Be a little more lenient when reading the cies/fdes. If next entry data
  winds up incorrect, don't fail, simply stop processing the entries. This
  only applies when reading all of the cies/fdes at once.
- Fail to init an eh_frame with no entries and fallback to assuming the
  eh_frame has no header instead.
- Change the step to always try debug_frame first which has the most
  accurate information.
- Add small unit tests and a couple of offline unit tests to verify
  this behavior.

These changes are needed to support offline unwinding since it depends
on this new behavior.

Bug: 65682279

Test: Ran new unit tests.
Change-Id: I3529f1b0c8e14cd7409494e5de2f3c9e78d0855e
diff --git a/libunwindstack/DwarfSection.cpp b/libunwindstack/DwarfSection.cpp
index 91d855b..04a1fae 100644
--- a/libunwindstack/DwarfSection.cpp
+++ b/libunwindstack/DwarfSection.cpp
@@ -835,9 +835,8 @@
     }
 
     if (next_entry_offset < memory_.cur_offset()) {
-      // This indicates some kind of corruption, or malformed section data.
-      last_error_ = DWARF_ERROR_ILLEGAL_VALUE;
-      return false;
+      // Simply consider the processing done in this case.
+      break;
     }
     memory_.set_cur_offset(next_entry_offset);
   }