Remove all field initializers and let ctor call clear().

The field initializers have basically caused the compiled <init> method
to inline the whole clear() method, which means if ProGuard is not used
or failed to inline or remove clear(), there are two big chunks of code
that do the same thing. So why not just call clear() from the ctor.

Change-Id: Ief71e2b03db2e059b3bfa98309649368089ffab0