allow priv_app self:process ptrace

The changes to ptrace in
https://android-review.googlesource.com/#/c/175786/ (removing it from
app.te and only adding it to isolated_app and untrusted_app) broke
WebView crash handling in cases where privileged apps (like gmscore) use
WebView.

The only way to fix this would be to allow priv_app to self-ptrace as
well. :/

Bug: 27697529
Change-Id: Ib9a3810dddc9f4213b6260133cbae23f669ae8dc