Refactor Android init methods for more flexibility

[mcginty]

After integrating rustls-platform-verifier into a codebase that uses Android, I wrote this PR to propose a more flexible initialization API. Specifically, it would be nice to, rather than passing in a context JObject that isn't Copy/Clone, to instead allow people to pass in a GlobalRef directly, which is what already happens behind the scenes in init_hosted.

I also modified the names of the existing methods as well to try to be more clear about what each of the 3 init options offer. I also kept the old names and marked them as deprecated to allow for backwards-compatibility.

Old	New
fn init_hosted(env: &mut JNIEnv, context: JObject) -> Result<(), JNIError>	fn init_with_env(env: &mut JNIEnv, context: JObject) -> Result<(), JNIError>
fn init_external(runtime: &'static dyn Runtime)	fn init_with_runtime(runtime: &'static dyn Runtime)
	fn init_with_refs(java_vm: JavaVM, context: GlobalRef, loader: GlobalRef)

[djc]

IMO it would be nice to split this into three commits, one for each rename and one for the new API. I'm not entirely sure whether it's worth it to force downstream users to go through a deprecation at this point for the cosmetic benefit of a better name -- maybe start with a soft deprecation as part of a comment?

[mcginty]

@djc good call, I like that plan better too. It also makes me think I can do a better job at documenting the differences in the rustdocs, so I'll take care of that and report back when they're broken into independent commits.

[mcginty]

Alright, done! How does this rendition look?

[djc]

This is much better!

My review is by no means sufficient for this code, but this looks okay to me.

[complexspaces]

This looks good, thanks for the addition. I built the docs locally to confirm the warning text shows up correctly as well:

[cpu]

Just a few small nits to note. Nice tidy :-)

[mcginty]

Pushed a commit with all the fixes for the typos mentioned in the review and while going through it one more time, realized that the warning block had some unnecessary commas and wasn't rendering markdown backticks as expected, so I just removed them.