Separate the definition of BTM layer types from control blocks

Right now, data types, control blocks, and functions used in the BTM
layer are defined in the same header files. This means that if someone
wants to write a test that uses those data types, they must also define
all control blocks, or compile the whole module.

This patch separates the data types from other definitions. Thanks to
it, we will be able to write unit tests, once other dependencies get
separated.

Change-Id: Ibc089e273cc37642fbb8672964b266c20f8d825d