[sanitizer] Reduce stack depot size on Android.

Summary:
The default setting kTabSizeLog=20 results in an 8Mb global hash table,
almost all of it in private pages. That is not a sane setting in a
mobile, system-wide use case: with ~150 concurrent processes stack
depot will account for more than 1Gb of RAM.

Reviewers: kcc, pcc

Subscribers: srhines, kubamracek, llvm-commits

Differential Revision: https://reviews.llvm.org/D56333

llvm-svn: 350443