Factor out calls to apexservice in a class.

The ApexManager class provides a cleaner interface to the apex service,
as well as providing caching for active packages, which can't change on
a running system. The cache is populated at boot time.

This CL will also cause PackageManager to stop reporting APEX packages
on devices that ship with flattened APEXs.

Test: atest apex_e2e_tests; used small app to verify API calls still
work; checked output of dumpsys.
Test: checked that on marlin (target with flatten APEX) no APEXs are
reported and no crashes are experienced at boot.
Fix: 123052859
Fix: 122638509
Fix: 124299505
Bug: 122952270
Change-Id: Iefe4fb42e455a7479ff47eb776d3492de8395469