Fix memory leaks

[kylecorry31]

Use LeakCanary to detect leaks and fix them

• [ ] Quick action buttons retain references to views (dispose on view destroyed)
• [x] Sighting compass retains references to views (dispose on view destroyed)
• [ ] Beacon layer retains references to canvas (dispose on finalize / destroy)
• [ ] Beacon compass layer retains references to canvas (dispose on finalize / destroy)
• [x] Main activity retained via context references (switch to app context)
• [x] Update timer should be destroyed when onDestroy is called