AidlComposer: read flag to determine number of readers

Follow-on to I28169352933dcc76ab9778a5a14a191867ea42f7. While that CL
doesn't (or at least shouldn't) change any user visible behavior,
it does slightly change the implementation. This CL makes it so that
the implementation only changes if the trunk stable flag (or debug
sysprop) is set.

Bug: 259132483
Bug: 284156408
Test: manual
Change-Id: I1ba3b4909129aba14036cfd34a5c3126b2483228