Protect dropbox service data with selinux

Create a new label for /data/system/dropbox, and neverallow direct
access to anything other than init and system_server.

While all apps may write to the dropbox service, only apps with
android.permission.READ_LOGS, a signature|privileged|development
permission, may read them. Grant access to priv_app, system_app,
and platform_app, and neverallow access to all untrusted_apps.

Bug: 31681871
Test: atest CtsStatsdHostTestCases
Test: atest DropBoxTest
Test: atest ErrorsTests
Change-Id: Ice302b74b13c4d66e07b069c1cdac55954d9f5df