Make PacProxyService be a system service

PacProxyInstaller class is running a thread all the time and is
listening to intent ACTION_PAC_REFRESH so it would be better to
make it be a system service with a manager class PacProxyManager
which is obtained with getSystemService(PacProxyManager.class).
Besides, rename PacProxyInstaller to PacProxyService will
be easier to know it's the service for PacProxyManager.

ConnectivityService is going to be a mainline module and it
needs constructor of PacProxyService to be SystemApi.
However, in current design, it needs to pass a handler and
an int arguments to the constructor which would be difficult
to maintain if just expose the constructor directly.

So, define a listener for the event that the current PAC
proxy has been installed so that the handler and the int
arguments can be removed from the constructor.

Bug: 177035719
Test: FrameworksNetTests
Change-Id: I2abff75ec59a17628ef006aad348c53fadbed076