Add scripts to install VNDK snapshot to prebuilts/vndk

update.py does the following:
1) fetches VNDK snapshot build artifact from Android
Build server,
2) installs snapshot under prebuilts/vndk/v{VNDK_VERSION},
3) autogenerates build files Android.mk and Android.bp
(using gen_buildfiles.py)

Test: python development/vndk/snapshot/update.py [options] &&
m -j vndk_v27
Bug: 69950927
Bug: 70125075
Change-Id: I81c8bd5060601a983da0c05bd63232b5631185b6