cts-tradefed: refer to frameworks/base for per-file backup OWNERS.

It didn't use to be possible to refer to another OWNERS file in per-file
rules, or at least the possibility wasn't documented. It seems to be
possible now, based on the examples given at frameworks/base/OWNERS.md.

After this CL, there is only exactly one OWNERS file remaining that
lists backup owners individually. All other OWNERS files now refer to
that one.

Bug: 159055442
Test: Treehugger
Change-Id: I5b764f4e4dab4b318d277f5afee16c05bb704d0f
1 file changed