Add read-write flag for optimized madvise readahead settings

This flag will control tuned improvements to the current madvise
readahead behavior. This includes:
    1) Using the POSIX_FADV_SEQUENTIAL fadvise hint
    2) Increasing the default madvise stride to 256kb

1) will unlock much larger kernel readahead performance, while 2) should
reduce the # of syscalls required and associated overhead.

Bug: 309384435
Test: local benchmarks w/ various large apps
Flag: com.android.art.rw.flags.madvise_optimized_readahead
Change-Id: Icde4a5ffc74dd8e4fdbcd5e1bb3b9eafa2b53655