expose the GetPidByName API of dumpstate

Bug: 32248316

This API is used by external dumpstate module
1) add unittest for GetPidByName
2) change the API name to c++ style
3) Test: dumpstate_test pass

Change-Id: I5eb80e0d1f1ef0f09b77ac3affd8a9b41d64f679
Signed-off-by: Ecco Park <eccopark@google.com>