[aapt2] Allow multi-line argument files

Previously, aapt2 would read arguments from files but they all have to
be in one line, separated by spaces. Now they can be spread over
multiple lines in the file.

Bug: 297373084
Bug: 315329624
Test: Called `aapt2 link ... --feature_flags @args.txt` with args.txt
containing flag values over multiple lines. Verified the resulting APK
has the expected output.

Change-Id: I91be6c7a869f63eafd88c04de69401d248f05545