Fix ProtectedMembersInFinalClass warning flagged by error prone

Fix ProtectedMembersInFinalClass warning flagged by error prone

Running recent error prone version complaining on that code:

CharacterHead.java:22: error: [ProtectedMembersInFinalClass] Make
members of final classes package-private: <init>
	protected CharacterHead(char expectedCharacter) {
	          ^
    (see https://errorprone.info/bugpattern/ProtectedMembersInFinalClass)
  Did you mean 'CharacterHead(char expectedCharacter) {'

Bug: 562756
Change-Id: Ic46a0b07e46235592f6e63db631f583303420b73
Signed-off-by: David Ostrovsky <david@ostrovsky.org>