Inherit base system_ext.mk files for aosp targets

Make aosp targets inherit handheld_system_ext.mk and
telephony_system_ext.mk files.
Devices that have /system_ext or /system/system_ext must inherit any
of *_system_ext.mk files to install mandatory packages for system_ext
partition.

Bug: 144542478
Test: Build aosp targets
Change-Id: If5752a12ccc5a414d54e480f9abe2b2db9eb9a2a