Add dependency on system API in connectivity

The system API are moved to the connectivity module: add dependency on
the stubs from framework-tethering.

This only allows tethering to depend on system API defined by the
connectivity classes, not on any private code, even if they are
eventually bundled in the same APEX.

Bug: 171540887
Test: m

Merged-In: Iebfb3e01c5d2480e84edcfc76cf489520bd60237
Merged-In: Ic2491f04880ff66abc3b2d3aaee6168e18f6d68a
(clean cherry-pick from internal branch)

Change-Id: Iebfb3e01c5d2480e84edcfc76cf489520bd60237