Reduce the memory requirements of a test

Equivalent change to upstream pull request #1888.

URLConnectionTest.testWrites allocates a 16MB buffer.
It can run in less memory (and causes problems
on low-spec Android devices).

4k is the typical smallest allowable buffer size on
Linux.

Bug: 24403364
Change-Id: I81e60c89b2463e9131c4a76521f9baa2187cdbda