Support animation parts fading out

This support helps with several things:
- simpler animation structure (size, number of items)
- more room for designers avoiding thinking about exit part
- time saving as complete parts now can be safely used as fading parts

Bug: 167352662
Test: Manual testing of major/edge cases
  - backward compatibility => "f 0" is mapped to "p", "p" still works
  - only trimmed areas are faded out
  - clock is not faded out (wear case)
  - adb shell su root bootanimation
  - [c, (exit) f 120, c] => "f 120" fades out and "c" completes
  - [c, (exit) f 120, f 320] => "f 120" fades out and "f 320" skipped
  - [c, f 16, (exit) f 120] => "f 16" played, "f 120" fades out
  - [c, (exit) f 0, f 120] => "f 0" skipped, "f 120" fades out

Change-Id: I9b200ee7107ef9b3dc8d711658ed1042b83739c2
Merged-In: I9b200ee7107ef9b3dc8d711658ed1042b83739c2