Increase default memory cache line size for android

Summary:
ADB packets have a maximum size of 4k. This means the size of memory reads does not affect speed
too much (as long as it fits in one packet). Therefore, I am increasing the default memory read
size for android to 2k. This value is used only if the user has not modified the default
memory-cache-line-size setting.

Reviewers: clayborg, tberghammer

Subscribers: tberghammer, danalbert, srhines, lldb-commits

Differential Revision: http://reviews.llvm.org/D13812

llvm-svn: 250814