vold: rename from "ext4 encryption" to fscrypt

We support file-based encryption on both ext4 and f2fs now, and the
kernel API is the same.  So rename things appropriately in vold:

    e4crypt => fscrypt
    ext4enc => fscrypt
    Ext4Crypt => FsCrypt
    EXT4_* => FS_*
    ext4_encryption_key => fscrypt_key

Additionally, the common functions shared by 'vold' and 'init' are now
in libfscrypt rather than ext4_utils.  So update vold to link to
libfscrypt and include the renamed headers.

Note: there's a chance of 'fscrypt' being confused with the dm-crypt
based encryption code in vold which is called 'cryptfs'.  However,
fscrypt is the name used in the kernel for ext4/f2fs/ubifs encryption,
and it's preferable to use the same name in userspace.

Test: built, booted device with f2fs encryption
Change-Id: I2a46a49f30d9c0b73d6f6fe09e4a4904d4138ff6
diff --git a/MetadataCrypt.h b/MetadataCrypt.h
index 841dc97..d82a43b 100644
--- a/MetadataCrypt.h
+++ b/MetadataCrypt.h
@@ -19,6 +19,6 @@
 
 #include <string>
 
-bool e4crypt_mount_metadata_encrypted(const std::string& mount_point, bool needs_encrypt);
+bool fscrypt_mount_metadata_encrypted(const std::string& mount_point, bool needs_encrypt);
 
 #endif