Clean up scan calls used for testing.

We've converged on explicit scanVolume() and scanFile() methods
which tests should be calling.  These are more robust than the
previous broadcast-based events, which could hang for a long time.

Bug: 127323913
Test: atest --test-mapping packages/apps/MediaProvider
Change-Id: I4f7918c70a67bcafab69ae2a71ee8e4bdaff01eb