fs_mgr: set ext4 encryption flag with tune2fs when needed

Upstream kernels (v4.9+, v4.4.67+) have started to enforce that
encryption policies cannot be set on ext4 directories unless
EXT4_FEATURE_INCOMPAT_ENCRYPT is set in the filesystem superblock, as
was the original design.  Since Android's userspace was not setting this
flag, it was not possible to use "file-based encryption" (FBE) on
devices whose kernels enforce this constraint.  Fix this by updating
fs_mgr to set the flag if needed, similar to how it enables the quota
feature if needed.

Note that it would, eventually, be simpler to set this flag at mkfs
time.  But that seems infeasible for now, given the many different ways
the userdata filesystem can be formatted --- including via 'fastboot',
which I believe is expected to still be compatible with old devices
whose kernel and/or e2fsprogs don't support the 'encrypt' flag.

Bug: 36231741
Change-Id: Ibafb9a7116fc853b62f8ee074a78499399f290a6