libkmpris: find active player index in filter proxy model

When the filter proxy model emits rowsRemoved, the player is still
not erased from m_container, so when the active player is the **last**
player in m_container, m_filterModel->mapFromSource(sourceModel->index(sourceRow, 0))
will return an invalid index because the source index becomes
stale (source_index.row() >= m->proxy_rows.size()) in the filter model.

BUG: 481992
FIXED-IN: 6.0


(cherry picked from commit 2f809bd7)