AAPT2: optionally keep resources without default value

Teach "aapt2 link" about a new flag --no-resource-removal. When given,
aapt2 will not filter out resources that lack default values. This is
useful mostly when building overlay packages that define resources for
non-default configurations, such as only for values-sv.

Test: manual: build package with resource only in values-vs, verify apk with aapt2 dump
Change-Id: Idc513bcb3f43bbff7f073163562c3dfccdb9bc9b
Merged-In: Idc513bcb3f43bbff7f073163562c3dfccdb9bc9b