Make PackageNameUserHandlePair Parcelable

For this, we need to move into a standalone class. We are also renaming
it to AppId.

This is a preliminary refactor for the implementation of volume sliders
for "Cast mirroring" sessions.

Bug: b/396394220
Test: atest CtsMediaRouterTestCases
Flag: EXEMPT refactor
Change-Id: I19504d00240178c0f1981eca34e6172e15abea7c