Add a 'Bluedroid conventions' section to the style guide.

Use booleans to represent boolean state, instead of a set of masks into an

integer. It's more transparent and readable, and less error prone.

## Bluedroid conventions

This section describes coding conventions that are specific to Bluedroid.

Whereas the _Language_ section describes the use of language features, this

section describes idioms, best practices, and conventions that are independent

of language features.

### Memory management

Use `osi_malloc` or `osi_calloc` to allocate bytes instead of plain `malloc`.

Likewise, use `osi_free` over `free`. These wrapped functions provide additional

lightweight memory bounds checks that can help track down memory errors.

By convention, functions that contain `*_new` in their name are allocation

routines and objects returned from those functions must be freed with the

corresponding `*_free` function. For example, list objects returned from

`list_new` should be freed with `list_free` and no other freeing routine.

## Header files

In general, every source file (`.c` or `.cpp`) in a `src/` directory should

have a corresponding header (`.h`) in the `include/` directory.