`rm -rf out` used to fail because bazel would not set write permission on some files. Now it's been fixed and thus there is little reason to prefer `m clean` (also users are accustomed to `rm -rf out`)
Bug: NA
Test: verified `m clean` works;interestingly soong_metrics file is created at the end
Change-Id: I000d16508613045811fc7792e5798f7c150dcc05
In particular, Bazel sometimes creates directories whose files cannot be deleted.
Test: m clean used to fail after b build //system/timezone/apex:com.android.tzdata, but now it works
Test: Added integration test which fails without this change
Change-Id: I08c8feed21c31ec187fe40be513f7eb4865c8ac3