Switch to UAST 1.3

This adds support for Kotlin 1.3, which also has the side
effect of switching packages for coroutine signatures.
This required a number of changes; for one thing, in
various places the PSI annotations would say @Nullable
for things like default parameters whereas the UAnnotations
correctly were marked as @NonNull, so metalava had to
start looking at UAnnotations instead.

Test: Unit tests updated
Change-Id: I9a7537a215ce3f8d065d6183bc5f09ebdf3bf77a