Restore android SoftReference javadoc

OpenJdk java.lang.ref.SoftReference javadoc
contradicts the Android version of its javadoc.
OpenJdk one encourages its use for caches, while
the old one says its a bad idea.

Old one makes most sense, this change adds it back.

Bug: 27441320
Change-Id: Ibef61741b92fe2d2f16ccd9bba358d07634f4dfb
1 file changed