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: Ibdbf0000ac4aa98c8485d67827f52208f9a827c5