Do not hold hash table lock while backtracing.

There is a deadlock if holding the hash table lock while trying to do
a backtrace. Change the code so that the hash table lock is only held
while actually modifying either g_hash_table, or while modifying an
entry from g_hash_table.

Bug: 22423683
(cherry picked from commit 9fee99b0)

Change-Id: I72173bfe6f824ceaceea625c24e7851b87467135