liblp: Make kDefaultGroup a string_view

Changing the type of kDefaultGroup from std::string to std::string_view
allows us to make it a constexpr object.  Since kDefaultGroup is the
only dirty object in liblp .bss at runtime, this change turns the .bss
page clean and thus saves 4KB per library load.

Bug: 138856262
Test: Boot cuttlefish and check liblp bss is clean for all 5 processes
      that are using it.
Change-Id: I7d7c0992e0ab769f070807f24e1275ffed424b5b