Use string length from DEX instead of recomputing

This gives around a 2% improvement in startup time overall. Here are results from a selection of
apps:

com.android.gallery3d/.app.GalleryActivity: 203.2ms → 197.7ms (change: -5.4ms, -2.7%)
com.android.messaging/.ui.conversationlist.ConversationListActivity:
  202.1ms → 199.1ms (change: -3.0ms, -1.5%)
com.android.contacts/.activities.PeopleActivity: 277.3ms → 270.7ms (change: -6.6ms, -2.4%)
com.android.camera2/com.android.camera.CameraLauncher:
  351.7ms → 344.1ms (change: -7.7ms, -2.2%)
com.android.dialer/.main.impl.MainActivity: 259.5ms → 254.2ms (change: -5.3ms, -2.0%)
com.android.settings/.Settings: 189.0ms → 186.5ms (change: -2.4ms, -1.3%)
com.android.email/.activity.Welcome: 222.8ms → 219.3ms (change: -3.6ms, -1.6%)
org.mozilla.firefox/.App: 370.2ms → 358.3ms (change: -11.9ms, -3.2%)

This is the average of 100 runs on a Pixel 2 XL.

Bug: 132691958
Test: device boots, start app many times
Change-Id: I93b6eb5105e32788cfc8159c6c21b400a161f86c
6 files changed