Add API to add custom close action for PiP.

Adds option to set a custom remote action to the PiP params to be used
instead of the system provided close action.

Although PipParamsBuilder gets a new setter, we do *not* add any new
getters to PipParams since the convention for this class has been to
not have any getters at all. The only existing public APIs are from
Parcelable: describeContents, writeToParcel, and CREATOR.

Bug: 218487423
Test: atest PinnedStackTests#testCloseActionIsSet

Change-Id: I04f1cc6177744e43236876cebbfc80cced1637ff