EncryptInPlace: ensure that backup superblocks get encrypted

Block groups with EXT4_BG_BLOCK_UNINIT still have backup superblocks
(and backup block group descriptors).  Fix EncryptInPlace to encrypt
these backup superblocks rather than leave them unencrypted.

Previously leaving the backup superblocks unencrypted didn't cause any
problems, but due to system/core commit 72abd7b246f7 ("Try to recover
corrupted ext4 /data with backup superblock") it is causing problems.

Bug: 162479411
Bug: 161871210
Change-Id: Ic090bf4e88193b289b04c5254ddf661ef40b037e