[scudo][standalone] Various improvements wrt RSS

Summary:
This patch includes several changes to reduce the overall footprint
of the allocator:
- for realloc'd chunks: only keep the same chunk when lowering the size
  if the delta is within a page worth of bytes;
- when draining a cache: drain the beginning, not the end; we add pointers
  at the end, so that meant we were draining the most recently added
  pointers;
- change the release code to account for an freed up last page: when
  scanning the pages, we were looking for pages fully covered by blocks;
  in the event of the last page, if it's only partially covered, we
  wouldn't mark it as releasable - even what follows the last chunk is
  all 0s. So now mark the rest of the page as releasable, and adapt the
  test;
- add a missing `setReleaseToOsIntervalMs` to the cacheless secondary;
- adjust the Android classes based on more captures thanks to pcc@'s
  tool.

Reviewers: pcc, cferris, hctim, eugenis

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D75142