Initialize an uninitialized enum.

frameworks/av/media/codec2/tests/C2UtilTest.cpp:82:39: error: variable 'e3' is uninitialized when passed as a const reference argument here [-Werror,-Wuninitialized-const-reference]
    C2FieldDescriptor::namedValuesFor(e3);
                                      ^~

Bug: http://b/155835175
Test: mma in frameworks/av/media/codec2/tests
Change-Id: Iba83877b6dbe17a922ad71988f13a853050ae7c8