Derive GridLayout's 'gap' constants from new "default_gap" entry in dimens.xml.

Also:

. removed static import of R.styleable - this is non-standard
. removed final modifier from DEBUG flag,
	this prevents the compiler from excluding the debugging code but
	allows debugging code to be enabled externally
. added override to removeAllViews() as GridLayout needs to be notified whever hiearchy changes.

Change-Id: I2409289bce7c6638eff320ffd48a6c0704e77fa8