liblog: fix android_log_printLogLine empty event string

Allow _one_ empty line to get through before evaluating
further. Add __android_log_bswrite testing for content
and contentless cases; checking for propagation and for
expected printing. As for printing which is fixed here,
security variants cover the same code paths as the events.

Bug: 26646213
Change-Id: I484718aa604e0a00afde4c34a00e87468ea93aa5