Rename PacManager to PacProxyInstaller

Generally, a manager class in Android is used to access
system services and it should be obtained from Context.
This class is a bit different from the definition of a
manager class.

API linter will detect an error if trying to expose a
class name end with Manager. ProxyTracker will create a
new instance of this class so this class needs to be
renamed to avoid API lint error.

Bug: 177035719
Test: FrameworksNetTests
Change-Id: I9185d4fb4342bd285a575f0bdd3518b758f37eb6