Expose ElementKey.currentAlpha() in ContentScope

This CL exposes a new ElementKey.currentAlpha() API in ContentScope that
can be used to get the computed alpha for a given element, reusing the
same logic used by Modifier.element() during transitions. This can be
used to share this logic with non compose elements like NSSL.

Bug: 437121774
Test: atest ContentTest
Flag: EXEMPT new API
Change-Id: Ic2bb2062f0f358309727a6c38545379b1d334f20