Remove references to non-std Java classes. [DO NOT MERGE]

This change adds a substitute for the Charsets class in java namespace.
The original cherry pick of the change had dropped this substitute. But
the substitute is required for bug b.android.com/59732

The original cherry-pick is 6335590a and
the original change is 81f74f4b
The original Change-Id is Iac5b272652e2780c9bb72d19f415d150948ca589

Change-Id: I76c87c74ee088778b37d1a7a7d531e0fde5c758f