feat(mdtest driver api): mdtest computes the maximum app-device coverage
score and generate app-device hitmap

mdtest is now able to report a app-device coverage matrix that shows
both reachable and unreachable app-device paths before test execution.
mdtest also reports an app-device hitmap based on the real execution
and spits the maximum app-device coverage score as well as the
percentage of app-device paths that are covered by the execution out of
all reachable app-device paths.  The hitmap as well as the app-device
coverage score helps users to discover under which app-device combination
some test executions fail.  It also tells the user which app-device
combination is tested and which is not.

mdtest also supports --brief flag which will only report test execution
output if set to true.

mdtest now does not support specifying test paths from test spec.  test
paths can only be specified from the command line.

Change-Id: I4b414d0f1efa17948a8b2846586f6e304a417e31
13 files changed