More LOW_4G optimizations

Eagerly try to map at an address when it doesn't overlap with
the ART known maps.

Reduces time spent creating the heap on N9 from ~100ms to ~10ms.

(cherry picked from commit 3ae250c1)

Bug: 20727525
Change-Id: Ie8982ca9d1f3158104b2b53b992c3a28841fc443