Support fraction type for inset* for InsetDrawable.

b/36024318

Test: manually tested on default icons.
Test: $ runtest --path=cts/tests/tests/graphics/src/android/graphics/drawable/cts/InsetDrawableTest.java

adb  shell am instrument -e class 'android.graphics.drawable.cts.InsetDrawableTest' -w 'android.graphics.cts/android.support.test.runner.AndroidJUnitRunner'

android.graphics.drawable.cts.InsetDrawableTest:........................

Time: 0.132

OK (26 tests)

Change-Id: Ie8d24680b6884e65017f07723a371120f5ae610c