Fix MediaOutputController resource management

This is a cherrypick of commit 42df638b.

MediaOutputController currently stops scanning when no
request to scan has been made yet, and also performs
unnecessary null checks.

Also remove dead code.

Bug: 232812007
Test: atest SystemUITests
Change-Id: I396d376a54b89c95a531b1499bd50be2e4242ed1
Merged-In: I396d376a54b89c95a531b1499bd50be2e4242ed1