Fix race condition with manual transitions

Transitions are triggered on background scope. Calls to
startTransition were transfered to the main dispatcher, but,
manual transitions that used updateTransition were not. This
could lead to a very rare race condition where a RUNNING step
could be sent after the corresponding CANCEL.

Test: atest KeyguardTransitionRepositoryTest
Fixes: 353698157
Flag: com.android.systemui.migrate_clocks_to_blueprint
Change-Id: I515358f0a9230d660fba4b0703eaad38477edd46