Don't set targetSdkVersion to '1' for libraries

Setting targetSdkVersion to '1' causes ManifestMerger to add
implicit permissions when merging to a higher targetSdkVersion.
It should really be unset, but ManifestMerger treats unset
targetSdkVersion as 'Q' if minSdkVersion is 'Q' (but not if
minSdkVersion is '28').  Set it to something low so that it
will be overriden by the main manifest, but high enough that
it doesn't cause implicit permissions grants.

Bug: 115415671
Bug: 117122200
Test: m checkbuild
Change-Id: I1d2d031a21314f6b55d8ea1cc7c4c8e3ecae7f06