Move sysfs_writable to emulator specific policy

This type is never used in core policy, only by emulators.
Move the definition of this type to where it's used.

Bug: 28221393
Change-Id: I38dbc12dbe9813f323d4bcd5f07679db57b2fd4a
diff --git a/target/board/generic/sepolicy/file.te b/target/board/generic/sepolicy/file.te
index 6fad80a..9227f80 100644
--- a/target/board/generic/sepolicy/file.te
+++ b/target/board/generic/sepolicy/file.te
@@ -1 +1,2 @@
 type qemud_socket, file_type;
+type sysfs_writable, fs_type, sysfs_type, mlstrustedobject;