liblog: replace "frontend" with "transport"

We still do not have any users of the 'frontend' interface, let's
right a wrong and rename it to 'transport' as it makes more sense.
Renames android_log_set_frontend, android_log_get_frontend and
include/log/log_frontend.h.

SideEffects: None
Test: gTest liblog-unit-tests
Bug: 27405083
Change-Id: I7c1c0f3dfdc7cf047285403e306edbd16ad1324d