Split generic_no_telephony in two.

Add handheld_system and handheld_vendor makefiles which
together make up generic_no_telephony. Document what belongs
in these files.

Also update mainline_system to inherit handheld_system.

Bug: 80410283
Test: lunch mainline_arm64; m nothing
Change-Id: Ic2f7b3dcaddc2c86a1187a394aebe8dfd86465ee
Merged-In: Ic2f7b3dcaddc2c86a1187a394aebe8dfd86465ee