[pidcat] Allow package to specify a process as well

Summary: Android apps can use multiple processes (eg, chrome), which are identified as <package_name>:<process_name>. The names
are defined in the manifest.
The main process is referred to as simply <package_name> by android.
Add the ability to specify a target process as part of what was until now the package name in pidcat, or all processes
if no process is specified.

Usage:
 * pidcat <package_name> will match all of an app's processes.
 * pidcat <package_name>:<process_name> will match a given named process.
 ** In particular, pidcat <package_name>: will ensure only the app's main/default process is matched.

Test Plan: Run the script, try out with no process, empty (aka default) process, named process. Try multiple entries.