Support a new ANDROID_AAPT_IGNORE env var.

AAPT has a fixed built-in list of files and directories
to ignore when parsing resource files. Over the years we
always had developers requiring specific patterns.
If the env var ANDROID_AAPT_IGNORE is set, it is parsed
to find which file/directory patterns to ignore.
Otherwise a default is used that matches the current behavior.

Added a command-line option for it:
  aapt di --ignore-assets "foo*:*.blah"

SDK Bug: 5343 24067

Change-Id: Ia4caa2a8188c8c1df143f884e459b8182645995f