Reorganize apex/apex.go

The source code is reorganized following the order of the execution for
better readability. The order is as follows:

1) init
2) properties
3) module struct
4) deps mutator
5) post-deps mutator
6) generate build actions
7) module initialization for different sub types
8) misc (e.g. apex_available check, etc.)

Behavior is not touched.

Bug: 173472337
Test: m
Merged-In: I919fca468fa376273d498029e6ac2fa37998a4a6
Change-Id: I919fca468fa376273d498029e6ac2fa37998a4a6