Use multidex capable application for chrome_public_test_apk.
https://codereview.chromium.org/2839983002 enabled multidex build to avoid the max limit of methods. But the tests app was not multidex aware on L and below devices (M and above working fine due to support in Android). Tests on waterfall were failing to find the needed classes.
BUG=
726516
Review-Url:
https://codereview.chromium.org/2905353003
Cr-Commit-Position: refs/heads/master@{#480537}
(cherry picked from commit 7ce5ca5baaa4efaa739ce6b5fe2444e010a5eb06)