Fix potential NPEs in PduParser.parsePartHeaders

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

Bug: 513581389
Test: m ApiDemos
Flag: EXEMPT BUGFIX
Cherrypick-From: https://googleplex-android-review.googlesource.com/q/commit:1e5089e05cc89c61fad92aa960f0659e1d6b1972
Merged-In: Ia8ea444778992ac7a3c2cdfab855e3b1f7e9c53f
Change-Id: Ia8ea444778992ac7a3c2cdfab855e3b1f7e9c53f