Do not modify input in-place

SortedUniqueStrings and FirstUniqueStrings dedupes repeating elements
and returns the deduped list. Currently, it also modifies the input list
in-place, which causes non-determinisitc failures like b/275313114

Operate on a copy of the input so that the input remains untouched.

SortedUniqueStrings is O(NlogN) and FirstUniqueStrings is ~O(N), so
creating a copy (O(N)) should not result in major performance regressions.
Numbers for this single unit test:
```
go test . -run TestStubsForLibraryInMultipleApexes -v -count 1000
Before: 174s
After: 172s
```

Bug: 275313114
Test: go test ./android
Test: go test . -run TestStubsForLibraryInMultipleApexes -v -count 1000
(cherry picked from https://android-review.googlesource.com/q/commit:8a8714c781175f8f1a6c189d919ee8b0ee8c1e27)
Merged-In: Id859723b2c2ebdc0023876c4b6fabe75d870bad7
Change-Id: Id859723b2c2ebdc0023876c4b6fabe75d870bad7