Merge "Trigger background memory trim when SystemUI is idle" into udc-dev am: d516a7caff

Original change: https://googleplex-android-review.googlesource.com/c/platform/frameworks/base/+/23130319

Change-Id: I488463bedfca155bb3d90fe4eac4e6f03c50fa31
Signed-off-by: Automerger Merge Worker <[email protected]>