gralloc4-vts: Hold fence by unique_fd to avoid leakage

There was fd leakage when fence was assigned to other values without
releasing previous file descriptor. The patch introduces unique_fd to
hold the fence without leaking fd.

Bug: 150461327
Bug: 152510209
Test: VtsHalGraphicsMapperV4_0Target

Change-Id: I926887ccd2c626da2d2c1a6b9d7cd433f9d5b717