Add option to avoid clobbering visibility of <declare-styleable>

"aapt2 compile" marks styleables as public instead of preserving
information from <public/> or --visibility options.  This behavior can
now be disabled via --preserve-visibility-of-styleables.

Bug: 146649511
Change-Id: Ifb8ab396573d1393df737a59625e79e9cf2494a7
Tested: aapt2_tests