Use "bootloader_prop" context for new "ro.boot.vbmeta.public_key_digest"

This property is set by init based on the corresponding "androidboot"
property set by the bootloader on the kernel commandline. It contains
the SHA-256 digest of the public key used to verify the vbmeta
partition (or boot partition if there is no vbmeta partition).

Bug: 220834466
Test: make
Change-Id: I83cfe0427ca9d79403785cf378157b9d31ffc1bf
diff --git a/private/property_contexts b/private/property_contexts
index b650fad..721f88b 100644
--- a/private/property_contexts
+++ b/private/property_contexts
@@ -1023,6 +1023,7 @@
 ro.boot.revision           u:object_r:bootloader_prop:s0 exact string
 ro.boot.serialconsole      u:object_r:bootloader_prop:s0 exact bool
 ro.boot.vbmeta.avb_version u:object_r:bootloader_prop:s0 exact string
+ro.boot.vbmeta.public_key_digest  u:object_r:bootloader_prop:s0 exact string
 ro.boot.verifiedbootstate  u:object_r:bootloader_prop:s0 exact string
 ro.boot.veritymode         u:object_r:bootloader_prop:s0 exact string
 # Properties specific to virtualized deployments of Android