[pm/incremental] remove LauncherActivityInfo.isStartable()

This API is not needed because any installed app is startable. See
go/incremental-when-to-freeze for detailed discussion.

BUG: 171299283
BUG: 171920377
Test: atest LauncherAppsSingleUserTest
Change-Id: I665b1005780babd02677a0f4ad8284ac81a5502e