Add nullability annotations to Fragment.
Add nullability annotations to Fragment.

For the simple acessors and mutators which already had obvious albeit implicit nullability.

Test: none
Change-Id: I17747ade52ab4361bd4f04ea092b6d32279c936f

androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx,aosp-mirror/platform_frameworks_support,androidx/androidx,aosp-mirror/platform_frameworks_support,AndroidX/androidx,AndroidX/androidx,androidx/androidx,androidx/androidx,aosp-mirror/platform_frameworks_support,androidx/androidx,aosp-mirror/platform_frameworks_support,androidx/androidx,androidx/androidx,AndroidX/androidx,AndroidX/androidx,androidx/androidx,aosp-mirror/platform_frameworks_support,androidx/androidx,AndroidX/androidx,AndroidX/androidx,AndroidX/androidx