Set optimize_for_size property for libLLVM_android

libLLVM_android is identified as one of the coldest system binaries with
significant file size through profcollect collection. Build with
optimize_for_size to reduce system image size.

Test: presubmit
Bug: 342090838
Change-Id: Ie273ac72d22a068fe95d185fe36703a9707f83ba