Change how memory policy is configured

Should be a mostly no-op but adds a handful of new options

* Adds a 'MemoryPolicy' to mostly consolidate memory settings
* Moves trim handling into HWUI proper
* Adds settings for UI hidden & context destruction that's not
  dependent on TRIM signals
* Treats persistent process the same as system_server
* Tweaks HardwareBitmapUploader timeout to reduce churn

Bug: 245565051
Test: builds & boots

Change-Id: I1f1b3db884ef7fa45ff2556436464a99440b998e