Add `MT_DEBUG_DEFAULT` system environment variable that turn on DEBUG by default

[mmathieum]

I want to be able to use MT_DEBUG_DEFAULT=true environment variable during Gradle project build on the computer to automatically switch org.mtransit.commons.Constants.DEBUG to true in the code.

Files:

• https://github.com/mtransitapps/commons-java/blob/master/build.gradle
• https://github.com/mtransitapps/commons-java/blob/master/src/main/java/org/mtransit/commons/Constants.kt