PDL: Test fixed-size struct fields

Add an end_index for bounds checking fields, and use that instead
of subviews, to support extraction in struct Parse() functions.

Bug: 138260498
Test: bluetooth_packet_parser_test --gtest_filter=*FixedTypesStruct*
Change-Id: I1e6819aafdd31d53de046299e7d17fe7dd2de197