Add API to add title and subtitle to the PIP.

On TV information about the PIP is to be shown in the dashboard and via
a card in the launcher. The app should be able to provide information
for this in the form of a title and subtitle if they want to.

Bug: 204173025
Test: atest PinnedStackTests#testTitleIsSet
Test: atest PinnedStackTests#testSubtitleIsSet
Change-Id: I805e010bad177215627f2a4876cd7bd7e0bd2314