Use get(0) instead of getFirst()

List.getFirst() was added in JDK 21. This will fail on Android versions
that don't support this (i.e. R, S, T) so fall back to using get(0)
which has the same functionality but works for all versions.

Fix: 422439135
Test: atest com.android.documentsui.picker.ActionHandlerTest
Flag: com.android.documentsui.flags.use_material3
Change-Id: Idc2d7417cfa6f6596b0836b23d8ce35e4d2658b5