Support gcov flushing in libhidlbase.

Create an additional sysprop callback for flushing gcov data when
available (guarded by ro.vts.coverage). Iterate through all libs loaded
into the process and call the flush function.

Test: vts_sancov_configure flush android.hardware.light@2.0
Bug: 67998360
Change-Id: Iad138d70f89cf3c0e68895c7dd0a7bf7207bd85d
1 file changed