Add some tools for testing ext4 performance and resiliency.

Add some tools for testing ext4 performance and resiliency.

rand_emmc_perf is a simple test to test random read/write performance
of emmc chips.  android_emmc_perf_tests is a script that runs on
the host that uses rand_emmc_perf to test the emmc performance on
a device.

corrupt_gdt_free_blocks is used to corrupt a filesystem so the kernel
trips over it at runtime and panics, thus testing kernel's ability to
mark the filesystem as needing to be fixed.

set_ext4_err_bit sets the error bit in the superblock so e2fsck will
trigger a full check next boot.

corrupt_gdt_free_blocks and set_ext4_err_bit are only built for
userdebug and eng builds.  rand_emmc_perf is marked optional, and
not included in any build by default.

Change-Id: I808174025d891f358ac54008371cb590e3c19f2f