Add support for checking lint on other files

This adds a new scope, Scope.OTHER, which can be used to indicate
that the check wants to investigate files other than the ones
specially handled as XML, Java, manifest or ProGuard files.
The PrivateKeyDetector now uses this such that it doesn't have
to perform its own directory iteration.

Change-Id: Ic38bf2db656f00c01d94c0c4e0112bc77218ea08
10 files changed