Move models json list into subdirectory.

We found that some devices add arbitrary
json files to the assets root. Moving models
list to seperate subdirectory to mitigate.

Bug: 109862575
Test: adb shell am instrument -w \
      com.example.android.nn.benchmark/android.support.test.runner.AndroidJUnitRunner

Change-Id: Id92b274e87dd41e89c6f6fd4f97ad9bf408c834f
Merged-In: Id92b274e87dd41e89c6f6fd4f97ad9bf408c834f
(cherry picked from commit cc612e0b1b0148688f9ae2c34549a633841cb4ee)
tree: c7aeceebeab75a113151dc3dc880a61a74fe58fa
  1. assets/
  2. LICENSE
  3. OWNERS
  4. README.txt