Make system property reading in LayoutlibLoader more DRY

The system properties can be stored in a std::unordered_map and can be
read in one fell swoop.

This reduces the amount of JNI boilerplate required to read individual
system properties.

Also remove the '**n/a**' magic string. LayoutLibLoader system property
logic is designed to support the empty string value.

This is effectively a cherry-pick of ag/25672148 but modified for
LayoutlibLoader in `main`.

Test: layoutlib-tests
Change-Id: I7562dc6eecada3199d8a3b20879b2e1b296cfb32