Fix fragment back navigation in PreferenceActivity with predictive back

When isOnBackInvokedCallbackEnabled=false, popBackStackImmediate() is called in Activity#onBackPressed. When isOnBackInvokedCallbackEnabled=true, we need to call popBackStackImmediate() manually when back is invoked, since onBackPressed is not called anymore.

Bug: 381779683
Test: Manual, i.e. verified back navigation in PreferenceActivity cts test app
Test: CtsPreferenceTestCases
Flag: com.android.window.flags.predictive_back_default_enable_sdk_36
Change-Id: I648362d320acb790802de143114f9c893e745335