feat(mdtest driver api): provide mdtest driver API that allows users to
access flutter drivers by device nickname

mdtest provides DriverMap class that helps users to access flutter
drivers through device nickname.  The API supports lazy initialization
which means the flutter driver instance will only be initialized the
first time the user wants to access it.  Singleton and factory pattern
is used to prevent redundant readings of the temporary spec file.

Add some code for previous submitted feature.  mdtest should now support
'os-version' and 'api-level' in the test spec.

Change code coverage report location.

Change-Id: I8a64629060071f993e15a64e6c08833211c1f96b
4 files changed