Combine madvise for clearing region space

Reduce how many madvise calls there are so that the mmap semaphore
has less contention.

Reduces the number of madvice calls from clearing regions when doing
few random camera gestures by ~80%.

Test: test-art-host
Bug: 62194020

Change-Id: I49987a371c5724e6993cfc74e385a03b624352c9