Replace IS_DEBUGGABLE with isDebuggable module API

Introduce a Build.isDebuggable() getter instead of the IS_DEBUGGABLE
static member.

Also make it module API as APK modules now have access to module APIs in
S, but also TestApi so it can be tested.

Test: m
Fixes: 182345786
Change-Id: I25d0848aee758792bbaba8ab73b5dd997a7250b3