getBlobAshmemSize -> getOpenAshmemSize

This API returns the total size of all ashmem memory that is in FDs that
is owned by a Parcel object. It was renamed a while ago, but the name
was never updated in Java.

Bug: 202029388
Test: atest android.app.NotificationTest
Change-Id: Icc428063083110952cf3951721d69cbb919429b1