CB-7536 check_reqs: windows tweaks + sdk manager error message

1. Don't escape \s since those are used by windows for directory seperators
2. Don't warn about missing directories on windows when we're just
testing for their existence
3. Don't give command to install sdk from command-line, since they also
require Build-tools and Platform-tools (which are not installed by
default with IDE-less SDK installer).
CB-7512 Change gradle android plugins from 0.10 -> 0.12
CB-7512 Add gradle environment vars for signing apks