liblog: minimum_log_priority should be atomic

In case multiple threads try to reference this variable while it is
being set, it should be atomic so that all threads always see a valid
value.

Bug: 150898477
Test: liblog, libbase unit tests
Merged-In: If6c9e291f2471b96a752dc6e76e3e63458b71391
Change-Id: If6c9e291f2471b96a752dc6e76e3e63458b71391
(cherry picked from commit bbb16022)