[jnigen] Change typeclass names and remove API leaks

[HosseinYousefi]

• Make Jni.env internal. We're not even using it in the JNIgen generated bindings. If users want to have a functionality here, they can open an issue. Making this internal heavily discourages use.
• Remove a degree of flexibility for type class naming, any class named Foo will have two typeclasses: $Foo$NullableType$ and $Foo$Type$ including the classes in package:jni itself.
• Remove the information about these symbols from jni_symbols.yaml (this will be changed again after jnigen transformation PR lands but wanted to land this first to make that PR a bit smaller)
• Same change in JNIgen, this fixes [jnigen] Inner class generates $Type class and mixin class with identical names #2493.
• Remove API leaks reported by the health bot ([jni] Fix the context when using multiple flutter engines #2494 (comment))
• Rename JObjType to JType and make it not internal. JType used to be the union of both primitive types and non-primitive types, however that is rarely used and internal so renamed that to JTypeBase and renamed the commonly used JObjType to JType.

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
jni	Breaking	0.14.2	0.15.0-wip	0.15.0	✔️

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API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbol	Leaking sources

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