Android: Fix ProGuard rules for BackgroundTasks

R8 is more strict than ProGuard, and does not keep any constructors
unless we tell it to.

Binary-Size: Growth due to correctly kept classes.
Bug: 985372
Change-Id: I6eaa2cc69e82ffa1400bdc1c10a5c348154d6616
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1709895
Auto-Submit: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Tommy Nyquist <nyquist@chromium.org>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679178}
2 files changed