Allow init to symlink userdata during first stage mounting.

In order to boot into GSI, we need init's first-stage block-device
machinery to find userdata. This will create its symlink before sepolicy
is loaded, leading to denials in the second stage.

Bug: 121209697
Test: device boots
Change-Id: Ibf3398c811016e09747116cf17393e8d22541bb2

public/init.te

@@ -43,6 +43,7 @@ allow init {
   misc_block_device
   recovery_block_device
   system_block_device
+  userdata_block_device
 }:{ blk_file lnk_file } relabelto;
 
 # setrlimit