Flip executorch android extension to source-only ABI (#23931)

Summary:

Flips 2 targets in xplat/executorch/extension/android to
abi_generation_mode = "source_only" (clean flip, comparator 0 on
both). Causation check: the single consumer-build failure
(etu2net_debug, missing aiplatform inference segmentation interface on
the stub classpath) reproduces identically with the flip reverted, so
it is a pre-existing gap unrelated to this change, not a regression.

Validation per target: buck2 build of default, class-abi and
source-only-abi variants green; KOSABI_APPLICABILITY_REPORT_V2 silent;
widened six-leg compareAbi reports 0 differences; KmDump shows 0
public member diffs; direct consumers build 239/241 with the sole
failure proven pre-existing as above.

Reviewed By: siaojiecai

Differential Revision: D125602727

Author: rybalkinsd

extension/android/BUCK

@@ -5,6 +5,7 @@ oncall("executorch")
 
 non_fbcode_target(_kind = fb_android_library,
     name = "executorch",
+    abi_generation_mode = "source_only",
     warnings_as_errors = False,
     required_for_source_only_abi = True,
     srcs = [
@@ -48,6 +49,7 @@ non_fbcode_target(_kind = fb_android_library,
 
 non_fbcode_target(_kind = fb_android_library,
     name = "executorch_llama",
+    abi_generation_mode = "source_only",
     warnings_as_errors = False,
     srcs = [
         "executorch_android/src/main/java/org/pytorch/executorch/extension/llm/LlmCallback.kt",