Move flattened APEX activation logic to apexd.

Also the check that linker configuration should not be updated if APEX
is in updatable state, is removed as now with refactoring of code and its
movement to apex project, that is also needed.

Resolves: https://b.corp.google.com/issues/192838499
Test: Run the AVD Device and check that it boots and run atest
ApexTestCases

Change-Id: I80a94fa0a4145d30546bb34a6d478a20a2881368