App crash fix when there is no internet connection

[jubeira]

This PR

• Fixes Map Nav when there is no internet connection (solving the address of the NTP server fails, so in that case we use the standard time provider) - Tested in network without internet. Closes #69 .
• Fixes Map nav directory structure
• Allows building the apps separately with Gradle Wrapper
• Downgrades the target SDK (API >=23 causes issues with the "shutdown" banner in Android M as the permission model changes).