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
13715d521f