Make SignalStrength constructor system APIs

Make SignalStrength constructor system APIs so that
it can be accessed by TelephonyRegistry

Bug: 140908357
Test: make
Merged-In: I80693b0f1b1f9de1a8c93ce8a23a816346afb634
Change-Id: I80693b0f1b1f9de1a8c93ce8a23a816346afb634
(cherry picked from commit 89e4e0c1e902e87d077443f4bf996f1529567092)
3 files changed