libprocessgroup: Use default member initializers

... instead of direct initialization in the default constructor
implementation. This allows us to just explicitly default the
constructor and provide no implementation.

Initialize class variables in the header and remove no-op constructor
to simplify code.

Bug: 346584259
Test: m
Change-Id: Iba57f5cfbf8e4d5d18ce178d39f8bcd2d76c8aaa