This reverts commit f23016c9.

this attribute is mandatory as per the XEP.

DIGEST and HMAC were static variables. Those are initialized by
what ever concrete implementation gets executed first.

(Perform SCRAM-SHA1 first and those variables got initialized with
SHA1 variants)

For subsequent SHA256 executions those variables contained wrong
values.

after removing the ability to select and copy text the transformation methods
provided by CopyTextView are no longer necessary.

fixes #3940

maybe fixes #3924

note that the ROMs I tested this on don’t require scanning for it
to appear on MTP. However it certainly don’t hurt either.

- 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.