Stop building unused dex.txt files

Testing with:
touch frameworks/base/core/java/android/view/View.java && time make -j framework

Before patch:
Run #1: 107.8 seconds
Run #2: 106.5 seconds
Run #3: 108.3 seconds
Average: 107.5 seconds

After patch:
Run #1: 105.5 seconds
Run #2: 105.6 seconds
Run #3: 105.0 seconds
Average: 105.4 seconds

Savings of 2.2 seconds (~2%)

Test: make framework
Bug: 151160048

Change-Id: Ic58875a315c9d1dc09b029d3de405a5f5648c46e