Move OnBackPressedDiser extension to main artifact

As the main activity artifact is automatically included
whenever a developer depends on activity-ktx and these are
in an atomic library group, we can move the Kotlin
extension from activity-ktx to activity itself now that
the class itself has been moved to Kotlin.

Relnote: n/a
Test: ./gradlew checkApi
BUG: 257291701
Change-Id: I13482de2e25bd6e084f4b299dddd43b2bcf7a4c7

• activity/activity-ktx/api/current.ignore[Added - diff]
• activity/activity-ktx/api/current.txt[diff]
• activity/activity-ktx/api/public_plus_experimental_current.txt[diff]
• activity/activity-ktx/api/restricted_current.ignore[Added - diff]
• activity/activity-ktx/api/restricted_current.txt[diff]
• activity/activity-ktx/src/androidTest/java/androidx/activity/OnBackPressedDiserTest.kt[Deleted - diff]
• activity/activity-ktx/src/main/java/androidx/activity/OnBackPressedDiser.kt[Deleted - diff]
• activity/activity/api/current.txt[diff]
• activity/activity/api/public_plus_experimental_current.txt[diff]
• activity/activity/api/restricted_current.txt[diff]
• activity/activity/src/androidTest/java/androidx/activity/OnBackPressedDiserTest.kt[diff]
• activity/activity/src/main/java/androidx/activity/OnBackPressedDiser.kt[diff]