Fix failure to load memtrack HAL implementation

Since some implementations of the libhardware memtrack HAL do not
implement an open() method, the HIDL implementation should not rely
on that method being available.

Test: Memtrack VTS tests pass
Bug: 31180823
Bug: 34103653
Change-Id: Ia0dda5e027894009bdcf12cd5c2e9eb635aca87e
Signed-off-by: Connor O'Brien <connoro@google.com>