private: hal_widevine_system: support private storage

- mediadrm_system_data_file file type
- make  it private to hal_widevine_system

Bug: 371777025
Test: lunch qemu_trusty_arm64-trunk_staging-userdebug
Change-Id: I5bd28eb4f2eaa44bb0b5a934b7919d2b959ec098
diff --git a/private/file_contexts b/private/file_contexts
index 23a895e..2bed8ed 100644
--- a/private/file_contexts
+++ b/private/file_contexts
@@ -596,6 +596,7 @@
 /data/system/ndebugsocket	u:object_r:system_ndebug_socket:s0
 /data/system/unsolzygotesocket  u:object_r:system_unsolzygote_socket:s0
 /data/drm(/.*)?		u:object_r:drm_data_file:s0
+/data/system/mediadrm(/.*)?		u:object_r:mediadrm_system_data_file:s0
 /data/resource-cache(/.*)? u:object_r:resourcecache_data_file:s0
 /data/dalvik-cache(/.*)? u:object_r:dalvikcache_data_file:s0
 /data/ota(/.*)? u:object_r:ota_data_file:s0