Decrease lock uses in RosAlloc::BulkFree().

Read rosalloc page map entries without a lock.

Disabled for now.

This change speeds up Ritz MemAllocTest by ~25% on host and reduces
the GC sweep time, but somehow slows it down by ~5% on N4, which is
why it's disabled for now. TODO: look into the slowdown on N4 more.

Bug: 8262791
Bug: 11790317
Change-Id: I936bbee9cfbd389e70d6343503bf0923865d2a2c