[ECM] Make AtomicInteger static

In a recent commit, I introduced an AtomicInteger to generate unique
request codes that should never repeat.

But, I mistakenly made it a instance variable. Make it static so it
will be unique across instances of this class.

Bug: 26056958
Test: atest CtsPermissionUiTestCases:android.permissionui.cts.EnhancedConfirmationManagerTest
Change-Id: I04d0b7f2be854cf84155811b15282a66bbc92199