Expose Build.VERSION.FIRST_SDK_INT as module-lib API

Connectivity service is going to become a mainline module which
will not be able to access hidden APIs. But PermissionMonitor
needs know Build.VERSION.FIRST_SDK_INT for granting network
restricted permission to system packages. Thus, expose the value
as module-lib API to support the usage.

Bug: 170598012
Test: make update-api
Change-Id: Id0ae42120f69faee43eeb7ebd35cae9e26cb7561