Fix flaky LoginWithCustomHighlightActivityTest#testAutofillCustomHighlight_multipleFields_hasHighlight

The MyDrawable sAutofilledBounds is a reference to Drawable mBound,
the mBound may be changed during the drawing, we should cache the
bounds of drawable in 1st draw not get reference of Drawable mBound.

Bug: 155586211
Test: atest --iterations 50 android.autofillservice.cts.\
LoginWithCustomHighlightActivityTest#\
testAutofillCustomHighlight_multipleFields_hasHighlight

Change-Id: I4e7a69230c08be5c495781381baf5457907e0e48
2 files changed