Fix idmap leak in zygote process

Fix a idmap leak in AssetManager::addSystemOverlays.
And, The fix could also prevent fd leak of idmap.

Test: none

Change-Id: Iff8831e1951a1ca103821f64a612a8b28d2c14e7
Signed-off-by: Hyangseok Chae <neo.chae@lge.com>