Modify build to put dumpcoverage agent into /system/lib

This location allows the agent to be attached to arbitrary processes.

It only makes sense to include libdumpcoverage.so on coverage builds,
as these are the only builds that have any information to be dumped.

Bug: 148178774
Test: manual, used examples in README (see other CL) to test whether
it works on a userdebug_coverage build on cuttlefish

Change-Id: Ib2fece1b41a3b5d16c8a2a444c5486137e475fda
Merged-In: Ib2fece1b41a3b5d16c8a2a444c5486137e475fda
(cherry picked from commit 7185f8cc)