GPU Memory: ensure bpf program is attached to the tracepoint

Previously GpuMem is gated upon bpfloader loaded. Recently, bpfloader
gets initialized much earlier which in return makes GpuMem initialize
earlier. However, at that time, the kernel gfx driver may still not get
loaded yet. This change fixes the racing here.

Bug: 159963505
Test: adb shell dumpsys gpu --gpumem
Test: atest gpuservice_unittest:GpuMemTest
Change-Id: If434c205ff3175354d85924075fc1de1e91151f9
Merged-In: If434c205ff3175354d85924075fc1de1e91151f9