core: Inline AbstractManagedChannelImplBuilder

[sergiitk]

Closes #7211.

Note to reviewers

Commit 75cccde preserves the order and style of AbstractManagedChannelImplBuilder in ManagedChannelImplBuilder, to highlight the changes necessary to inline the superclass:

git diff c1d6a09~:core/src/main/java/io/grpc/internal/AbstractManagedChannelImplBuilder.java c1d6a09:core/src/main/java/io/grpc/internal/ManagedChannelImplBuilder.java

And the changes made to tests (mostly unchanged):

git diff c1d6a09~:core/src/test/java/io/grpc/internal/AbstractManagedChannelImplBuilderTest.java c1d6a09:core/src/test/java/io/grpc/internal/ManagedChannelImplBuilderTest.java

The following commit `cc0185c` organizes the methods and properties in `ManagedChannelImplBuilder` after inlining the superclass: 1. Package-private mutable state used for building`ManagedChannelImpl` is separated from private states and constants 2. Methods implementing `ManagedChannelBuilder` preserve the order they're defined in `ManagedChannelBuilder` 3. Private and `@VisibleForTesting` methods moved next to where they are executed or fit logically.

[ejona86]

There's still a lot of changes to the AbstractManagedChannelImplBuilder code. I suggest you start with AbstractManagedChannelImplBuilder and modify it with pieces of ManagedChannelImplBuilder.

[ejona86]

As an example, I see noise like:

-  /**
-   * An idle timeout larger than this would disable idle mode.
-   */
+  /** An idle timeout larger than this would disable idle mode. */

And movement of lines like:

-  private static final Logger log =
-      Logger.getLogger(AbstractManagedChannelImplBuilder.class.getName());