Add genrules for merged system server stubs

This is a change analogous to https://r.android.com/1432681 for system
server stubs; it feeds into metalava the stubs for the non-updatable
system server api stubs and the individual mainline module system server
stubs to generate a merged .txt file that will eventually replace the
monolithic one in frameworks/base/services/api/current.txt .

Bug: 177640454
Test: diff frameworks/base/services/api/current.txt \
out/soong/.intermediates/frameworks/base/api/services-system-server-current.txt/gen/system-server-current.txt

Change-Id: I021f59bb91572d28798a97ab8aa687b684599d6c