Avoid getMemory binder calls when using default Memtrack HAL

The default Memtrack HAL implementation doesn't support the getMemory
method. Avoid making binder calls to this method in MemtrackProxy if the
default implementation is being used.

NO_IFTTT=Updated in ag/33020719

Test: atest memtrackproxy_test
Flag: EXEMPT bugfix
Bug: 388928701
Change-Id: Ieea4fdd59fa9c399c097b95f33442ef10ce9eb3f