Adds a method to check if there's an NNAPI accelerator available. Bug: 141353636 Bug: 137150012 Test: CtsNNAPIBenchmarkTestCases Change-Id: Ic0d066f29b1b0b4d10c7f9105dd342aca27408cf Merged-In: Ic0d066f29b1b0b4d10c7f9105dd342aca27408cf (cherry picked from commit 5e82b1b805a378504872e3724121c857c2eccc6b)