Use Makefile variables to specify margin settings.

Instead of defining device-specific UI class, this CL allows using
Makefile variables to specify margin values directly.

Values explicitly defined via TARGET_RECOVERY_UI_MARGIN_HEIGHT and
TARGET_RECOVERY_UI_MARGIN_WIDTH will be used. Otherwise they will
default to zero.

Bug: 62732748
Test: Specify the height and width and check recovery texts.
Change-Id: Icb6f7466c8d407f877b93da38aebfdf7e6b41be7