Use option to flush the entire buffer on -EOVERFLOW

This was the previous behavior prior to project system/media commit
fc3d70736e4c24cbdc8c91076013c01e487fd4d5.
The new behavior introduced by that commit is to preserve as much of the
buffer as possible.  But that introduced a regression in applications with
so many overflows that they can't recover.  So now the flush is optional.
In the case of PipeReader, we do need the flush.

Test: at bug
Bug: 36725187
Change-Id: I25609c7a656a05228fb4e7d4082b2857f4195a34