Base: Add more log macros

Add WOULD_LOG to determine whether a given severity would be logged.

Add LOG_STREAM to have direct access to a logging stream.

Add LOG_S variants that take a fully qualified severity. This allows
complex expressions as parameters, e.g., ternaries for conditional
severity levels.

Add tests.

Bug: 31338270
Test: m
Test: mmma system/core/base && $ANDROID_HOST_OUT/nativetest64/libbase_test/libbase_test64
Change-Id: I242b960594e68caff6db9cd8aaa4ce8aaf90474c