Remove exportable modules when generating snapshots targeting older platform

This change modifies the contents of the generated Android.bp files so
that when generating a snapshot on a older platform, the "exportable"
modules are removed from the bp files, as the "exportable" modules are
first introduced in V and do not exist in older platforms.

Bug: 345162614
Test: ABTD
Change-Id: I2dba51b98deec7805bd796647a66981f237c55a9