Use android.os.Debug.getCallers instead of self implementation.

This changes debug message format but does not change any other behavior.

Maintaining InputMethodUtils.getStackTrace() doesn't make much sense
because we already have a similar method in android.os.Debug.

since there is already utility function in Android. The length 10 is
sufficient for identifying caller modules.

getStackTrace() is introduced by
Ib23849d352db33f0747aa9d5a178f00ac726c13b just for debugging purpose as a
part of Bug 6931482.

Bug: 22285167
Change-Id: If3a6c0d2f51dc3774eef14b4f496b5b59ddcf5a6