Add formats for declare-styleable attributes

AAPT does not allow for attributes declared in declared-styleables that
do not have a format to create new attribute resources. AAPT2 does and
should not.

Declare-styleable attribues added while AAPT2 was enabled and that are
missing a format definition must declare a format.

Bug: 131100106
Test: build
Change-Id: I3affed34e6a534e70b159faee26a16964070b133