Unexpected size bloat of the APK

[yukawa]

Environment

• Mozc 2.18.2553.103 (52e3d29) and later

Steps to reproduce

1. Build Mozc for Android (release build)
2. Check the file size of android/bin/MozcForAndroid-release-unsigned.apk

Expected behavior

The file size is roughly 18MB (or so), which was the size as of Mozc 2.18.2552.103 (7b9ea33)

Actual behavior

The file size is roughly 30MB.

Additional information

Before 52e3d29 was committed, assets/mozc.imy was embedded if and only if use_separate_dataset GYP variable is set to 1. With 52e3d29, C++ layer started behaving as if use_separate_dataset was always set to 0, which is intentional, while assets/mozc.imy has always been embedded as if use_separate_dataset was always set to 0, which is never intentional.