Add code coverage report support to Bluetooth

Allow the generation of code coverage reports for any code that has
native tests.

In order to add coverage support to code do the following steps.

1. Add "clang_file_coverage" as a default to the source library(s) you
   want statistics for.
2. Add "clang_coverage_bin" as a default to the host supported test.
3. Add the test name and the files you want covered to "gen_coverage.py"
    - The files covered can either be a directory or files.
    - Avoid using / at the beginning of a covered file/dir as this
      breaks how the coverage generator resolves filenames.

Bug: 114751344
Test: run ./gen_coverage.py and test arguments manually
Change-Id: I9dd04f6ee2fd642531e301e3bfd52713f862cd4b