liblogcat: free up log_device_t structures

Was leaking log_device_t in command path.  Cleanup leak in command
path and add thorough clean up on destroy for insurance.

Start grouping related like-type variables in the context structure
for more effective layout.

Test: gTest logcat-unit-tests
Bug: 35326290
Change-Id: Ibfbddec2d0e1bce24b87b035d67726cac1395574