Fix the error code ANEURALNETWORKS_UNMAPPABLE

  - The error codes should not clash with each other.
  - Change ANEURALNETWORKS_UNMAPPABLE to be of value 7.
  - The impact of this enum value change is expected to be minimal.

Bug: 68356625
Test: make checkbuild
Test: NNAPI CTS and VTS tests pass.
Change-Id: I85a8b9a2180659d9057564119802d851e4011fc6
Merged-In: I85a8b9a2180659d9057564119802d851e4011fc6
(cherry picked from commit 2dcdbd97)