Add apksigner tool to Android O SDK build-tools

This places apksigner command-line tool in Android SDK's build-tools
directory. The tool is intended as an Android-specific replacement for jarsigner.

Bug: 62696222
Change-Id: I099ea6840d0e7e0b00f438357543cb656990442a
(cherry picked from commit a825e5d309629b2f9cb3b52ba6d9895562729001)