Implement AppIoBlocked counter per uid

This allows us track AppIoBlocked status correctly for a uid with
several threads concurrently blocked on IO.

This would have been cleaner to track per tid of uid, but
unfortunately, we don't know the calling tid via binder

Test: Manual
Bug: 170486601
Change-Id: I4df5cb2e99726739059d962915c7488df98a0f05