Improve handling of external intents

Use ENTRY_ACTIVITY from !1087 (merged) as a central router for external intents:

Not signed into Briar. The password screen is shown, then after signing in the remote contact activity is shown, as expected. The contact's link field is empty. If I use the up button to navigate away from the remote contact screen or the pending contact list, the task is closed. I'd expect to navigate to the contact list when pressing the up button. If I don't use the up button to navigate away, then later reopen Briar from the foreground notification, a second Briar task is created. I assume the same would happen with private message notifications, etc - the issue being that the notification expects us to have a task that can be reused by clearing the stack down to NavDrawerActivity, but the existing task doesn't have that activity in its stack.

There's also some other issues with AddContactActivity to resolve:

If an instance of AddContactActivity already exists it's brought to the foreground but the link isn't populated (I'm assuming this is because the new intent is delivered without calling onCreate() again).

If I open the remote contact activity via the speed dial, then put Briar into the background via the home button and relaunch via the ongoing notification, the contact list appears as expected. But when I back out, the remote contact activity and another instance of the contact list are underneath.

Similarly, if I open the remote contact activity with a share intent, then press the home button, relaunch via the ongoing notification, and back out of the contact list, the remote contact activity is underneath (but without the second instance of the contact list).

Finally, if I open the remote contact activity with a share intent, then back out or press the up button, the task is cleared. If I relaunch via recent apps, the remote contact activity is shown again. Backing out or pressing the up button clears the task again, so I can't reach the rest of the app!