service: Remove local copies of base and modp_b64

Since libbase is now available on AOSP (under libchrome) Bluetooth code can
depend on it rather than maintain its own copy a subset of base sources (and
modp_b64 dependencies).

Dependent CL: http://r.android.com/158335

Bug: 22175181
Change-Id: I3a301e49353598633ad857ac4345d0361c273beb