Add lock when iterating over BacktraceMap.

In order to avoid a race condition in the maps data, acquire
a read lock while iterating over the maps data.

Bug: 29387050
Bug: 31067025

(cherry picked from commit 56f8b5631ab2874dc2ec385802e38555acac0d90)

Change-Id: I48ecd2705b4aa4eedfed1daae2eb3b9b8bf8dda9
1 file changed