Revert "Lose ls to toybox."

This reverts commit 3536bfb8.

Current cts and tradefed rely on toolbox ls output for file navigation
and using toybox's causes the following tests to fail:

com.android.cts.aadb.TestDeviceFuncTest#testSyncFiles_normal
com.android.cts.aadb.TestDeviceFuncTest#testSyncFiles_extStorageVariable
org.apache.harmony.tests.java.lang.ProcessTest#test_exitValue
org.apache.harmony.tests.java.lang.ProcessTest#test_getErrorStream

Change-Id: I121439e53a64df2abe9306262f1e742c408d82b5