Fix use-after-free crash in VtsHalNeuralnetworksTargetTest.

Prior to this CL, the AHardwareBuffer in TestBlobAHWB is released in the
destructor, but later used (unlock) during the destruction of the
mMapping member. This CL fixed this issue by managing the lifetime of
AHardwareBuffer with SharedMemory.

Bug: 197199690
Test: VtsHalNeuralnetworksTargetTest
Change-Id: I00748aaaa1a3a3d9b3b62bedb77a655ddb6e210f