Make generate_gradle.py work with preview SDKs.

This just tweaks the values of buildToolsVersion and compileSdkVersion,
which were not being set in a way compatible with preview SDKs.

BUG=620034

Review-Url: https://codereview.chromium.org/2724513003
Cr-Commit-Position: refs/heads/master@{#453737}
2 files changed