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
Change-Id: I6e3c4122d5d618b8a9ac054c19b0a3d19e7d3b20
7 files changed