mediaprovider: fixed sharedUserId bug

All apps signed with the media key share a UID. However,
some run in the priv_app selinux context, and others run
in the mediaprovider context. That's a bug. Apps which share
a UID should always share an selinux domain. Assign all apps
with the seinfo=media to the mediaprovider selinux domain.

This moves the following packages from the priv_app to the
mediaprovider domain:
com.android.providers.downloads
com.android.providers.downloads.ui
com.android.mtp
com.android.soundpicker

Bug: 154614768
Test: atest CtsDownloadManagerApi28
Change-Id: I6f96142ef03101568abed670a0e32f952515a590
diff --git a/private/seapp_contexts b/private/seapp_contexts
index 87e8b83..6c86bdb 100644
--- a/private/seapp_contexts
+++ b/private/seapp_contexts
@@ -153,7 +153,7 @@
 user=webview_zygote seinfo=webview_zygote domain=webview_zygote
 user=_isolated domain=isolated_app levelFrom=all
 user=_app seinfo=app_zygote domain=app_zygote levelFrom=all
-user=_app seinfo=media domain=mediaprovider name=android.process.media type=app_data_file levelFrom=user
+user=_app seinfo=media domain=mediaprovider type=app_data_file levelFrom=user
 user=_app seinfo=platform domain=platform_app type=app_data_file levelFrom=user
 user=_app isEphemeralApp=true domain=ephemeral_app type=app_data_file levelFrom=all
 user=_app isPrivApp=true domain=priv_app type=privapp_data_file levelFrom=user