[pm/incremental/api] exposing package state names and reasons

We would like to expose package states and unstartable reasons to public
APIs. Two main users will be:
1) sys UI, which will display alert windows on top of a foreground app
when the package becomes unstartable, with different contents in the alert
window based on the reason code.

2) Play Store, which will collect package state change information which
will be used to determine future package delivery strategies.

Test: will add CTS tests.
BUG: 162600626
CTS-Coverage-Bug: 168925819
Change-Id: I71bafb36b230ea93b34e8556b57563e4ba3b837d