SurfaceFlinger: reset the FB_TGT acquire fence

This change makes SurfaceFlinger reset the acquire fence fd to -1 after each
call to HWComposer::commit.  The HWComposer implementation is resonsible for
closing the fd, so SurfaceFlinger should make sure not to pass it to HWC
multiple times.

Change-Id: I79554d9f6d6ef2b77d632d40251516c1f5b16ddb
Bug: 7258954