Add extra filegroup for telephony related sources

This filegroup is used to obtain extra mappings in the
hiddenapi-mappings target.

Test: m hiddenapi-mappings
Bug: 110868826

(cherry picked from commit 740f28fe)

Change-Id: Iedc489842dad99099173f5087b0f94a90641ae8a
Merged-In: I0771603eeaa7c597b730802d2d8d40efcfff827c