Handle stylus buttons and tool types.

Added TOOL_TYPE_ERASER.

Refactored the InputReader to share more code between the
various input mappers that handle button states and to
simplify the accumulator implementations by having each
one only handle a single type of input.

Removed the concept of direct/indirect tool types from the API.
If we add it back, it should be done in a manner that is orthogonal
to the tool type itself, perhaps as a flags field on the pointer.
The device source may well provide sufficient information anyhow.

Change-Id: I811c22d95e8304269b6ee4f6d11a6b04f3cfc1b2