Reuse existing build id strings.
Reuse existing build id strings.

With https://r.android.com/1505492 this will avoid constructing a new
string for the Build ID for every unwind, if we have an AllocRecord to
reuse.

Bug: 169842943
Change-Id: Ifbf5fd762ed5e7f01f108e347bd00552e6c1c113
google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto,google/perfetto