Use framework-res keep rules for system_server

The framework-res manifest defines a number of services implemented in
system_server. These required manual keep rules to prevent shrinking,
but now we can use the aapt-generated keep rules to achieve the same
result, without the maintenance burden.

A follow-up change will trim the existing keep rules to remove redundant
keeps, and plumb the same keep rules to other targets that define
system_server jars.

Bug: 272495195
Test: m + validate no net change to services.jar
Change-Id: Ibbb3ba47417ecadfae92764933761c4339ca6b35