Set targetSdkVersion in manifest_fixer

If targetSdkVersion is not set in the manifest, set it to the
value it was implicitly using before changing minSdkVersion.
Requires passing --library to manifest_fixer.py to distinguish
between apps, where the implicit value was set by aapt2 to
current, or libraries where the implicit value was 1.
Fixes cases where the manifest does not specify targetSdkVersion
and was inheriting the minSdkVersion value until manifest merger
started merging a lower targetSdkVersion value from a library.

Bug: 111347801
Test: manifest_fixer_test.py
Change-Id: I8fcf0c5f452707565ba1808f6fe552ffed055c47