libprocessgroup: Allow vendor profile attributes to override system ones

In the current implementation vendor profile attributes do not override
system ones and instead generate a warning. Fix that by overriding existing
attribute if a new definition is found.

Bug: 160318642
Test: add vendor attributes and confirm no warnings
Signed-off-by: Suren Baghdasaryan <surenb@google.com>
Change-Id: I71a2ee4d4b3c585e7c9a01b791e973390d409cbc