Lint tool for current.txt.

Detects common public API mistakes, making it easy to quickly scan
across all changes in a new API level.

Builds in-memory representation of the current API, then runs various
detectors over the classes.  It collects failures, and can ignore
known failures from a previous API level.

Example usage:

$ python frameworks/base/tools/apilint/apilint.py \
    frameworks/base/api/current.txt \
    prebuilts/sdk/api/21.txt

Change-Id: Ie1f88894baf9f79e4b11e78c47f848ea5e25fc25