Fix GC map generation edge case

Previously we checked the number of bits, but then rounded up to a
byte boundary. If ref_bitmap_bits was 65535, it would pass the check
but not actually fit in the 13 bits for ref_bitmap_bytes since we
rounded up to 8192 bytes.

(cherry picked from commit 1626b919)

Regression test infeasible due to smali limitations.

Bug: 21888453
Change-Id: If4b769431f27ecca14fe7852d017bcde01b3e146