Fix potential NPEs in PduParser.parsePartHeaders

Add null checks for contentDisposition, filename, and
Content-Transfer-Encoding headers to prevent NullPointerExceptions.

Bug: 513581389
Test: m framework-minus-apex
Flag: EXEMPT BUGFIX
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:a13540708f1b5a6870faf76b75c545a7650649a9
Merged-In: I4cad171d30b14f3b6fd9dd2a0ef6bb1e5320665d
Change-Id: I4cad171d30b14f3b6fd9dd2a0ef6bb1e5320665d