Allow launching placeholder when the TaskFragment is visible

Before, we only launch placeholder from the top most
TaskFragmentContainer. Now we also launch placeholder from the one
that is not the top most, but is visible. This can happen when the
top most is transparent.

Bug: 261550242
Test: atest WMJetpackUnitTests:SplitControllerTest
Merged-In: I6e3c4122d5d618b8a9ac054c19b0a3d19e7d3b20
Change-Id: I6e3c4122d5d618b8a9ac054c19b0a3d19e7d3b20