gn-build: Get btif to compile as its own module

gn-build: Get btif to compile as its own module

This CL adds support to build //btif:btif as its own target as a static library
by introducing a new module-level BUILD.gn file, adding missing includes, and
conditionally compiling code that is Android-specific (depends on libcutils).
This removes/breaks some functionality on non-Android builds since the
corresponding features are missing until we have a more unified replacement.

BUG=21339022

Change-Id: Ibb0372da45f256c55fa894fdda3d70319ea008c9