rename services-stub-sources filegroup to services-all-sources

The filegroup contains all source files for the system server. Calling
it services-stub-sources is misleading and is causing a confusion with
'services-stubs.sources' which is the metalava-filtered source files for
the stub library 'services-stub'.

Bug: N/A
Test: m
Change-Id: I123fbee3c5b5f4d3e8e070e4374d40effc6b799c