Add Build.IS_DEBUGGABLE to public API

For testing purposes, system components such as mainline modules may
need to relax some security restrictions on debuggable builds only, in
cases where there is no testing strategy that would be reasonable on
user builds security-wise.
One example is to test interfaces between a mainline module and the
rest of the system, which typically require very privileged permissions.

There is no particular reason or permission check to keep the symbol
non-public so add it to the public API.

Test: m
Fixes: 163699557
Change-Id: I15c093af04f024eed5567151b241a4bc35c611d4