Pass WRITE usage flag to the mapper if ImageReader was created with a

WRITE usage flag.

Since some producers rely on these buffers being unchanged (eg. for
partial rendering), this feature should be used with caution. A warning
to this effect was added in the Javadoc.

Bug: 162287616
Test: built emulator, confirmed no SIGSEGV anymore (b/162287616 fixed).
Change-Id: Ia1f8b8754b0d6ae75a2af33147645e744d593d82