Update StrictMode's public API.

This makes it more future-proof and maintainable, not exposing the
internal bitpacking state.

The implementation is unchanged (the policy is still just an int we pass
around).

Also starts to introduce VmPolicy, for things which are process-wide,
not per-thread.  As an initial user, make SQLite's Cursor finalization
leak warnings use StrictMode.

Change-Id: Idedfba4e965716f5089a52036421460b1f383725