first_stage_init: Optimize redundant vector copy

```
std::vector<std::string> v2;
for (auto&& e : get_temporary_v1()) { v2.push_back(e); }
```
^^^ This is constructing v2 from a temporary object, which is an
unnecessary manual clone. We should just let copy-elision do its thing:

```
// Give compiler the chance to optimize this with copy-elision...
auto v2 = get_temporary_v1();
```

Also `lp_names` appends an extra ',' at its end. Just use
android::base::Join() here.

Bug: 235111004
Test: Presubmit GSI boot test
Change-Id: Ibe8ce7a29b8521e789aa0a99f7f6d31f3f9c70e9