Performance of m for AOSP

USE_NINJA=false

• GNU make parses and evaluates Android.mk (~1m30s)
• GNU make executes necessary commands

USE_NINJA=true (first build)

• kati parses and evaluates Android.mk and generates build.ninja (~30s)
• ninja parses build.ninja (~3s)
• ninja executes necessary commands (optionally with goma)

USE_NINJA=true (second build)

• kati finds it doesn't need to re-generate build.ninja (~1s)
• ninja parses build.ninja (~3s)
• ninja executes necessary commands (optionally with goma)