fs_mgr: stop allowing the FDE fstab options

Since Android 10, new devices have been required to use FBE instead of
FDE.  Therefore, the FDE code is no longer needed.

Make fs_mgr reject fstabs where FDE is enabled.

Unfortunately, there is a quirk where the "encryptable" flag (which was
originally meant just for FDE) was overloaded to identify adoptable
storage volumes.  It appears that we have to keep supporting this use
case.  Therefore, don't reject the "encryptable" flag completely.
Instead, just reject "encryptable" when it appears without
"voldmanaged", or without "userdata" as its argument.

Here are some references for how "encryptable=userdata" is being used to
identify adoptable storage volumes:

  * https://source.android.com/devices/storage/config#adoptable_storage
  * https://cs.android.com/android/platform/superproject/+/f26c7e9b12e05a6737a96b44bada77232e08ed87:system/vold/main.cpp;l=269
  * https://cs.android.com/android/platform/superproject/+/f26c7e9b12e05a6737a96b44bada77232e08ed87:device/google/cuttlefish/shared/config/fstab.f2fs;l=17
  * https://cs.android.com/android/platform/superproject/+/f26c7e9b12e05a6737a96b44bada77232e08ed87:device/generic/goldfish/fstab.ranchu;l=7

[ebiggers@: modified from a WIP CL by paulcrowley@]

Bug: 191796797
Change-Id: I3c4bbbe549cc6e24607f230fad27ea0d4d35ce09