Update the default and the min feature level in NNAPI runtime.

Update the default and the min feature level to 8 because FL8
will be shipped with the next dessert release and there is no
mendel experiment ramping up needed.

Cherrypicked from I284efa740f8041e845e21f249ef1948e8228a98d

Test: NeuralNetworksTest_static
Bug: 209987514
Bug: 205927215
Change-Id: I284efa740f8041e845e21f249ef1948e8228a98d
Merged-In: I284efa740f8041e845e21f249ef1948e8228a98d