PDL: Make `endianness_declaration` mandatory

Since we need to know the endianness to parse anything, we now make
the declaration mandatory. This matches the behavior of
`bluetooth_packetgen`.

Bug: 229953170
Test: atest packages/modules/Bluetooth/tools/pdl
(cherry picked from https://android-review.googlesource.com/q/commit:2aff9cb955dca1768c1683b856f764464dbfcc59)
Merged-In: I7a48a72ec0fcf53d304f036e18282602668f2b17
Change-Id: I7a48a72ec0fcf53d304f036e18282602668f2b17