liblog: __android_log_pmsg_file_write() cleanup

__android_log_pmsg_file_write() will open /dev/pmsg0 if not
already, and will close it if we opened it.

Added atomic access to the android_log_context as insurance.

Fortify and correct pmsg tests.

Test: gTest liblog-unit-tests --gtest_filter=liblog.__android_log_pmsg_file_*
Bug: 31958686
Change-Id: I2cf6f971b6968938f471fda67367efe20dae3004