Add documentation on how lints are defined and used in Android. Merge
the deny_warnings attribute with a new attribute (no_lint) which can be
used to disable the default linting parameters.
Explicitly allow all lints for external/ and prebuilts/, which remove
any warning when building sysroot for the devices.
Test: cd external/rust/crates; mma
Test: add dummy internal Rust module; mma
Change-Id: I62be1c41aeda4068fb9e288038727c1de5ffe547
