Support WIC.show(IME) from Activity#onCreate

In Q, app developer would've to wait for InputConnection on Editor before
showSoftInput() could be called. This made API very un-intuitive. While
WIC.show(ime) took care of some of the scenarios, it didn't handle
showing IME from onCreate() i.e. when control is not yet available.
This CL introduces a pending IME show request that waits until IME
control is available.

Test: atest WindowInsetsControllerTests#testShowImeOnCreate
Bug: 153014086

Change-Id: I93dd5790e1fb3d7841ad1323a45cd3bae2d6e9b8