Dispose bindings on rebind

Without this, coroutines were remaining active and memory
was leaking. Also, reduce the number of callbacks added to
KeyguardStateController to 1.

Test: perfetto + ahat
Bug: 344990605
Flag: com.android.systemui.migrate_clocks_to_blueprint
Change-Id: If1ed7fa20d1d8b3cb685b1e4c1ea2030b39ab55c