core, alts, cronet: fix ByteBuffer covariant method usages

[voidzcy]

Java 9 introduces overridden methods with covariant return types for the following methods in java.nio.ByteBuffer:

• position​(int newPosition)
• limit​(int newLimit)
• flip​()
• clear​()
• mark​()
• reset​()
• rewind​()

In Java 9 they all now return ByteBuffer, whereas the methods they override return Buffer, resulting in exceptions like this when executing on Java 8 and lower:

java.lang.NoSuchMethodError: java.nio.ByteBuffer.limit(I)Ljava/nio/ByteBuffer

This is because the generated byte code includes the static return type of the method, which is not found on Java 8 and lower because the overloaded methods with covariant return types don't exist (the issue appears even with source and target 8 or lower in compilation parameters).
The solution is to cast ByteBuffer instances to Buffer before calling the method.

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See details in #7348.

Fixed with find-replace, I can't guarantee not missing one or two. Tweaked with animal sniffer, but seems it doesn't catch it.

[ejona86]

Could you make sure to add more information to the commit description? The issue is useful for all the nitty-gritty, but a short summary or pointer would be helpful directly in the commit.