Add libtimeinstate for reading uid time-in-state BPF map

Add a library to simplify reading the per-UID time_in_state data
collected using eBPF.

Also add some basic gtests to test the provided interface

Bug: 78498733
Test: gtests pass on crosshatch after loading time_in_state bpf
program
Change-Id: I997e0fd2faad398c7b427084a08edacb9405f089
Signed-off-by: Connor O'Brien <connoro@google.com>