Enable the android:largeHeap flag

- With large accounts (such as mine), Conversations starts hitting up against
  the default heap limit pretty quickly, at which point it grinds to a halt as
  GC pause times increase.
- Furthermore, it's impossible to complete a backup with such an account, since
  Conversations will just run out of memory before the backup can complete.
- Enabling the `android:largeHeap` flag asks the OS for a bit more memory, which
  hopefully alleviates the problem for larger accounts.