dynsystem.SparseInputStream: Implement read(buf, off, len)

InstallationAsyncTask calls SparseInputStream.read(buf, off, len),
however SparseInputStream didn't override that method, so the method
actually fallbacks to the default implementation, which is a for-loop
calling read() (single byte). This is a significant performance hit.

This modest fix boosts the DSU installation time significantly:
* physical device: 45s -> 15s
* virtual device: 30s -> 7s

Bug: 225310919
Test: start a Dynamic System installation
Change-Id: I5a61dc2e9554719aece43491b813f6647b4bcc65