Improve assertions to verify elements, not size

Switch more assertions from using size and isEqualTo(0)
or isEmpty() and isTrue() to using the appropriate
assertions to check the exact elements or size.

Test: existing tests still pass
BUG: 147749580
Change-Id: Ie194694f60420e2c98b140c59b5d548acdf097c3

• fragment/fragment/src/androidTest/java/androidx/fragment/app/FragmentTestUtil.kt[diff]
• fragment/fragment/src/androidTest/java/androidx/fragment/app/FragmentTransitionTest.kt[diff]
• fragment/fragment/src/androidTest/java/androidx/fragment/app/PostponedTransitionTest.kt[diff]
• transition/transition/src/androidTest/java/androidx/transition/FragmentTestUtil.kt[diff]