`std::erase` is a replacement for the remove-erase idiom.

Changes to `OpenModeToAndroid` inadvertently revealed that the prior implementation had UB (potentially deleting the end iterator). This is now fixed.

Previously we were translating "w" to "w", which unlike in C++ doesn't
truncate. See https://issuetracker.google.com/issues/180526528, and for
reference, https://en.cppreference.com/w/cpp/io/c/fopen and
https://en.cppreference.com/w/cpp/io/basic_filebuf/open.

This issue was brought to my attention by the recently published issue
CVE-2023-21036 in the screenshot editing tool on Pixel phones. I'm not
aware of any code in Dolphin that actually uses "w" with an existing
file on Android (when we ask the user for a location to save to using
SAF, a new file is always created), but still, best to fix this.

Several of these can take a string_view rather than needing to
specifically use std::string.

Obsoletes these functions in favor of the standard member functions
added in C++20.

If we're in a function that isn't just going to immediately return to
Java, leaking local references can lead to problems eventually.

Now it also works when b isn't at the very end. (+ goes after b.)

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.

Any local references get cleaned up when returning to the JVM,
but some of the functions in AndroidCommon return to C++ rather
than the JVM, and functions with loops risk running into the
limit of how many simultaneous local references are allowed.

The Java implementation of getting the list of post-processing
shaders only looked in the Sys folder and not the User folder.
This could be fixed in the Java implementation, but it's
simpler to just call the C++ implementation instead.