Stop preventing scans when the screen is off

- Companion apps need to be able to scan even when the screen is off.
- The uid importance should be sufficient to enforce that (non-companion)
  apps don't scan when they are not in the foreground (or the screen is
  off).

Bug: 304234628
Test: atest MediaRouter2HostSideTest CtsMediaBetterTogetherTestCases
Change-Id: Ie1ddc6b4ca9c0fc313cb2ee209603d3338e67f6a