Reland of 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.

Reverted in:
https://codereview.chromium.org/2722973002/

Reason for reland:
Fixed up non-absolute path.

TBR=estevenson@chromium.org,nyquist@chromium.org
BUG=620034

Review-Url: https://codereview.chromium.org/2727523002
Cr-Commit-Position: refs/heads/master@{#453827}
2 files changed