Keep aidl generated C++ inside the generated folder

Transform ../ to dotdot/ for C++ generateds from .aidl source files.
This forces us to use one layer of indirection to calculate the build
rules for .aidl files, since we can no longer use a pattern rule.

This was tested by modifying system/tools/aidl's Android.mk to refer to
its .aidl files by going up two directories and then repeating the
directories again.  When I print the build rules with $(info) I see that
dotdot/ appears in appropriate places (C++ paths, but not .aidl paths).

Bug: 26407018
Test: Described above.

Change-Id: I397c9d10408c0c66d8b5a247a1f34eb4bf4f74ce