Fix flaky predictive back test

The reason for the flakiness was that `onBackProgressed` was not called sometimes because the progress was already at 0f. Setting the progress to 0.5f ensures that it is animated back to 0f and thus `onBackProgressed` is definitely called.

Bug: 366251353
Test: WindowOnBackInvokedDispatcherTest
Flag: EXEMPT bugfix
Change-Id: Ia09330f54f6a61245183ce10d762b97931c76245