Allow overriding DeviceDiagnosticsPreferenceController.

Allow devices to specify their own flags by overriding this class.
Additionally, fix a missing PackageManager lookup in getIntent().

Bug: 348647186
Test: launch Settings on Pixel and on AOSP
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:287181169485eb30203c7b698b1f8ee618059e29)
Merged-In: I2489a64804f068a957f9be3f87847d6911b24447
Change-Id: I2489a64804f068a957f9be3f87847d6911b24447