Header elevation pixels

[ryandt]

• Pass in a pixel value instead of a DP value for header elevation to match consistency with Android's View.setElevation() method and to allow clients to use a value from their app's dimens file so that elevation dimensions can be reused/standardized more easily.

• Remove elevateHeaders() to avoid differing behaviors for calling both elevateHeaders() and setHeaderElevation(). For example, as a client, is setting setHeaderElevation(0) the same as `elevateHeaders(false)? Exposing only one method to add or remove elevation avoids the confusion and potential conflicting behavior outcomes.

• Remove the cachedElevation field and elevation resolution helper methods since they're no longer necessary.