Use a randomly generated UUID as the TvInputSessionId

The current implementation uses uid, pid and the resolvedUser to create
the sessionId. But it's not unique when the same app uses the same
TvInputService to create multiple input sessions.

Also this leaks pid/uid information.

Fix this by using a randomly generated UUID instead.

Test: make
Bug: 163855754
Change-Id: I4369e2645b50d44803f2d393798fa5ea6d1c22bd