memtrack: eliminate dependency on surfaceflinger in VTS test

VTS tests need to be able to run without the framework or native
servers. Call getMemory() on the test process instead of on
surfaceflinger to facilitate this.

Bug: 35945712
Test: Test passes on marlin and bullhead.
Change-Id: Ie847da708a95360ed5f2b444973e45b091e912a0
Signed-off-by: Connor O'Brien <connoro@google.com>