Added AppStartFullyDrawn metric to AppStartupHelper.

Note that this metric is a raher limited one as it replies on the app
calling reportFullyDrawn() and few apps do that.

Bug: 120227235
Test: AppStartupHelperTest.java; note that this cannot be run from AOSP.
Instead, copy this into internal and run it according to instructions
within the file.
Change-Id: I7cec76472deeb402b7481882c74d342052dc4d56