Bug 6165: Do not omit java.lang prefix in various places

Bug 6165: Do not omit java.lang prefix in various places

Yang expression "typedef String" leads to java class String,
creating potential naming conflict with java.lang.String.
Few places places in EnumTemplate are fixed by using «String.importedName».

Also, YangModuleInfoTemplate#getExplicitType
no longer removes java.lang unconditionally.

+ Ifs against void are re-ordered to occur sooner in getExplicitType.

Change-Id: If014aca30875589e361733317dfb47eaa07df031
Signed-off-by: Vratko Polak <vrpolak@cisco.com>