Base: Add INTERNAL_FATAL to logging severity

Add the INTERNAL_FATAL level. It will print 'F' like FATAL, but
does not abort after logging.

Add a test to logging_test.

Bug: 31338270
Test: m
Test: mmma system/core/base && adb sync && adb shell /data/nativetest/libbase_test/libbase_test32
Change-Id: Idf74c08e8516881efccaefc58fa3f41d57e56396