RootCanal: Replace GD logger by libbase logging

The GD logger in RootCanal wasn't working for VERBOSE and
DEBUG log levels because of the absence of InitFlags

libbase logging implements a Google-style C++ logging interface
which is easier to use in C++ than printf style specifiers

libbase is also available for the host so we use that instead
of the GD logger

To ease the migration and define a logging interface for rootcanal
a log.h header have been introduced to shim the old GD logger
style macros

The minimum log level on the host can be choosen via the
ANDROID_LOG_TAGS environement variable, for example
ANDROID_LOG_TAGS=*:v set the minimum log level to verbose

Test: Run root-canal
Change-Id: I88a33bc010b0be4724ef5956f9a664245c1d4e7e