Remove computeValue.isSpecified

This CL removes the isSpecified lambda from Element.computeValue. This
is not necessary anymore since http://aosp/2978852, which was pulled to
our prebuilts in http://ag/26476625.

Bug: 316901148
Test: atest PlatformComposeSceneTransitionLayoutTests
Flag: N/A
Change-Id: I668ee3694f17d610f5e151e560aa2012bd73d618