Add basic implementation of ShadowProcess

ShadowProcess can be used when mocking android.os.Process.myPid().

I found a identical class from the android git repository.

https://android.googlesource.com/platform/external/robolectric/+/df5951775e9c13526de56166ccdb841aa910a674%5E/src/main/java/com/xtremelabs/robolectric/shadows/ShadowProcess.java