Update android_compile_arm_rel base builder in binary-size.config

android_compile_arm_rel was migrated from arm32 to arm64 in
https://webrtc-review.googlesource.com/c/src/+/469921 and
android_compile_arm64_rel was removed as redundant in
https://webrtc-review.googlesource.com/c/src/+/470260.

Update binary-size.config so android_compile_arm_rel is compared against
Android64 Builder arm64 instead of the 32-bit Android32 Builder arm, and
remove the decommissioned android_compile_arm64_rel entry.

Bug: webrtc:427152624
Change-Id: I5ee2ba4bd04cb0acdea46cf5a55f9e56a1bed2f3
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/508560
Reviewed-by: Jeremy Leconte <jleconte@google.com>