Simplify NNAPI SharedHandle -- hal

Prior to this CL, the NNAPI represented SharedHandle as a collection of
file descriptors and a vector of ints to act as metadata. This struct
mirrored Android's native_handle_t. However, the NNAPI only uses
SharedHandle to hold a single file descriptor to hold a single cache
file.

A sibling CL simplifies the SharedHandle by changing it to be a single
ref-counted file descriptor. This CL makes changes to the HAL utility
code in response to the SharedHandle change.

Bug: N/A
Test: mma
Test: NeuralNetworksTest_static
Change-Id: Ib390ad76bcdadffd85f8c2c502189aaa7da483e4