Ensure flicker split test activities handle resize

- SplitScreenActivity launches SplitScreenSecondaryActivity with
  LAUNCH_ADJACENT in onPostCreate(), but after entering split the
  activity will be resized and recreated (since it doesn't handle
  config changes), which results in SplitScreenSecondaryActivity
  being started again (which is subsequently blocked from launching
  on the next side due to recent changes, and instead launches on top
  of SplitScreenActivity)

Bug: 344216031
Test: atest UnlockKeyguardToSplitScreenGesturalNavPortrait
Flag: EXEMPT bugfix
Change-Id: Ie657de49a460da2c61a9563ca6c051a922c4e764