[5/N] Remove the ability to select the Legacy OomAdjuster impl

Flag: EXEMPT pure refactor
Test: atest MockingOomAdjusterTests
Test: atest ServiceBindingOomAdjPolicyTest
Test: atest OomAdjusterTests
Test: atest ActivityManagerProcessStateTest
Test: atest ActivityManagerFgsBgStartTest
Test: builds and flashes
Bug: 306731120
Change-Id: I0a6d321966bcc67d5ba22734a6363db82d506262