Nr 423904 redefinition of a session with no longer calling agent shutdown on background

[mbruin-NR]

This PR introduces the NRMASessionManager class, which manages the mobile session lifecycle for the New Relic Agent. It enforces a 4-hour maximum session duration and a 30-minute background timeout.

Key Changes

• Added NRMASessionManager as a singleton for session management.
• Tracks session start, last activity, and background timestamps.
• Implements logic for session expiration based on duration and background timeouts.
• Provides utility for current time in milliseconds.
• Includes debug logging for session events.