GpuMemTest - switch mTestMap bpfmap init to move assignment

also remove use of .getMap() which I want to remove as it exposes fds
and can leak (isValid check in the next line is equivalent)

Bug: 236285127
Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ib5e1430acba423fa3d0373833411d44fe7cdde2a