Remove MediaManager and push down callbacks to InfoMediaManager

MediaManager's only implementation was InfoMediaManager and there were
no direct usages of MediaManager outside of a few tests. The CL also
removes unused fields.

This is a non-functional change.

Bug: 327195340
Flag: NA
Test: Presubmit + atest com.android.settingslib.media
Change-Id: I7d7a11f47fc35358d9b8dd22e2d014b49a360263