Adapt ReadableMapBuffer to MapBuffer interface

Summary:
Updates `ReadableMapBuffer` to conform to `MapBuffer` interface, to allow interchangeable use of `Readable/WritableMapBuffer` in the code.

Notable changes:
- MapBuffer.Entry getters are now represented as Kotlin properties and appended `Value` suffix (e.g. `entry.getInt()` becomes `entry.getIntValue()` in Java, or `entry.intValue` in Kotlin)
- `ByteBuffer` is imported in constructor instead of on demand. This method doesn't copy the data as the bytes are read directly from native heap, and benchmarking a 500-byte `MapBuffer` shows no difference in import speed.
- Internal logic of `ReadableMapBuffer` uses `UShort` kotlin type for key retrieval, for more correct representation of values.
- Explicit exception throws are replaced with `require` and `check` methods for `IllegalArgumentException` and `IllegalStateException` (default FB conversion).

The change also updates `ReadableMapBuffer` usages to `MapBuffer` interface where possible (virtually everywhere except JNI methods).

Changelog: [Android][Changed] - Adopt `MapBuffer` interface for `ReadableMapBuffer`

Reviewed By: mdvacca

Differential Revision: D35218633

fbshipit-source-id: 515dd974c27b2978ade325b2e1750ab8f068a20a

ReactAndroid/src/main/java/com/facebook/react/fabric/BUCK

@@ -14,6 +14,7 @@ rn_android_library(
         "pfh:ReactNative_CommonInfrastructurePlaceholde",
         "supermodule:xplat/default/public.react_native.infra",
     ],
+    language = "KOTLIN",
     provided_deps = [
         react_native_dep("third-party/android/androidx:annotation"),
         react_native_dep("third-party/android/androidx:core"),
@@ -21,6 +22,7 @@ rn_android_library(
         react_native_dep("third-party/android/androidx:legacy-support-core-ui"),
         react_native_dep("third-party/android/androidx:legacy-support-core-utils"),
     ],
+    pure_kotlin = False,
     required_for_source_only_abi = True,
     visibility = [
         "PUBLIC",