Only keep default constructor of manifest items

Reflection-based instantiation of manifest-declared types is only done on the default (aka no-argument) constructor. While these types are unlikely to have alternate constructors (unlike the others listed in the bug), there's no reason to generate overly keep-y rules for them.

Bug: 37123156
Test: make aapt2_tests
Change-Id: Ic83a2671a54ea5dd558bfcacf033a60e9568ab8c