Break out static update method

Simply getting package settings could changes stored state. Break out
the majority of the method to modify local variables and not change
any stored state. The top-level getPackageLPw() method will still
mutate stored state. This will be changed in a future CL.

Also add a set of tests to verify the behaviour of updatePackageSetting()

Bug: 30219944
Change-Id: I3360a36ce238e816246ee8ca7ecabfbbcdf0b89d