Fix backup MAC checking.

[elkhadiy]

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Description

if(MessageDigest.isEqual(ourMac, theirMac) was always returning false since ourMac was of length 32 and theirMac was of length 10.

10-30 21:20:02.045 6562-6609/org.thoughtcrime.securesms D/MAC_CHECK: theirMac = C16A82AB25ACD6292668
10-30 21:20:02.046 6562-6609/org.thoughtcrime.securesms D/MAC_CHECK: ourMac   = C16A82AB25ACD6292668C5C4DFE3C8B3405EC4E87DC87E5B67B22536A68F8248

This issue most likely went under the radar since a ! was forgotten in the condition.

I'm also not sure of what happens when an exception is thrown in case of a MAC missmatch to the output stream in readAttachmentTo(OutputStream out, int length). There is a out.close() before the check, the file is written even if an exception is raised ?

[greyson-signal]

Tweaked slighty and merged in 787bcf7. Thanks!